What Is an Invoice? Types, Key Elements & Get Paid Faster

Most small business owners send an invoice and wait. No reminder system, no payment terms conversation, no clear due date language: just a PDF attached to an email and a hope it gets paid. That wait has a real cost. According to QuickBooks' 2025 Small Business Late Payments Report, 56% of US small businesses are currently owed money from unpaid invoices — $17,500 each on average — and 47% have at least some invoices overdue by more than 30 days. Many of those invoices are never collected at all.

This guide covers what an invoice is, what to include in one, the 13 types and when to use each, and practical steps to manage your invoicing process so you get paid on time.

It draws on official IRS guidance, Generally Accepted Accounting Principles (GAAP) invoice standards, and common invoicing practices used by US SMEs and international businesses. Wherever a process or rule may vary by jurisdiction or contract type, we have noted it.

An invoice is a document a seller sends to a buyer to request payment for goods or services delivered. It records the transaction details: what was provided, the agreed price, and when payment is due. From the seller's perspective, an invoice creates an accounts receivable entry. From the buyer's perspective, it creates an accounts payable obligation.

Invoices are legal documents. If a buyer disputes a payment or a seller needs to pursue a debt, the invoice is the primary record both sides rely on. A well-constructed invoice, with clear payment terms and an itemised list of goods or services, gives you standing to chase payment and, if necessary, escalate to a debt collector or court.

Every invoice should include the following information. Missing any of these makes it harder to get paid and harder to chase payment if something goes wrong.

Here is what a completed invoice looks like in practice:

The colour coding shows which fields stay fixed across every invoice (business name, logo, bank details), which usually remain consistent (buyer information, service descriptions), and which change with each new invoice (date, invoice number, amount, due date).

💡 Tip: Write out the due date as a specific calendar date (e.g. 30 May 2026) rather than "30 days from invoice date." Relative dates leave room for disagreement about when the clock starts, and date format differences (05/06 means different things in the US and Europe) create additional confusion in international transactions. A full written date removes both problems.

There are 13 types of invoices, each suited to a different transaction. Routine sales, international trade, long projects, refunds, and subscription billing each have a dedicated format. 

Here is a summary of each type, followed by a detailed breakdown of when and how to use it.

A standard invoice is the default format for requesting payment after goods or services are delivered. It includes all the elements listed above: seller and buyer details, a line-item list, the total amount due, and payment terms.

➡️ When to use a Standard Invoice

For most business-to-business and business-to-customer transactions. If no other invoice type applies, use this one.‎

Example: A graphic designer completes a logo project for a client and sends a standard invoice for the agreed fee, listing the service, the rate, and the payment due date.

A commercial invoice is used for international shipments. Customs authorities in both the exporting and importing countries use it to assess duties and verify that the goods comply with import regulations. It must include the country of origin, a detailed description of the goods, their value in the relevant currency, and the Harmonized System (HS) code for each item.

Failure to include a compliant commercial invoice can hold goods at customs or result in additional charges. For a full guide on what a commercial invoice must contain, see our commercial invoice guide.

➡️ When to use a Commercial Invoice

Any time you ship physical goods across international borders.

Example: A manufacturer in Los Angeles ships 600 machine parts to a buyer in Canada. The commercial invoice lists each part, its unit price, the total value, and the HS code so Canadian customs can assess the applicable duties.

A pro forma invoice is a preliminary document sent before delivery to outline what the final invoice will look like. It is not a payment demand and carries no legal obligation to pay. Its purpose is to give the buyer enough information to approve the order, arrange financing, or open a letter of credit.

Businesses in international trade often use pro forma invoices to help buyers estimate import duties before goods arrive. The document looks like a standard invoice but is clearly labelled ‘Pro Forma Invoice.’

➡️  When to use a Pro Forma Invoice

Before you commit resources to an order. If you are manufacturing to specification, importing goods, or bidding on a contract, a pro forma invoice confirms the agreed terms without creating a legal payment obligation on either side.‎

Example: A machine parts supplier receives an enquiry for 60 precision parts. Before manufacturing begins, they send a pro forma invoice so the buyer can confirm the specifications and price.

A past-due invoice is any invoice that has not been paid by the payment due date. When a payment becomes overdue, the seller reissues the invoice with a note that it is past due, including the number of days overdue and any late fees that now apply under the original payment terms.

➡️  When to use a Past-due Invoice

Automatically, when a payment due date passes without settlement. Issue the past-due notice promptly: delays in follow-up are the single biggest reason late invoices go uncollected. For a step-by-step process on what to do next, see our guide on how to chase outstanding invoices.

Example: An invoice with a Net 30 payment term was issued on 1 March. By 1 April, no payment has arrived. The seller sends a past-due notice referencing the original invoice number and adds the applicable late fee.

A retainer invoice requests advance payment for future work. It is most common in professional services: legal, accounting, consulting, and creative work. A retainer invoice is typically issued alongside a retainer agreement that defines the scope of work and how unused retainer funds are handled.

➡️  When to use a Retainer Invoice

When a retainer agreement is in place. Issue the retainer invoice at the start of each billing period — before work begins, not after. This protects you if the engagement ends mid-month.

Example: A client hires a law firm for ongoing legal support. The firm issues a retainer invoice each month for the agreed retainer amount. Work completed is billed against the retainer, and a final invoice is issued if costs exceed it.

An interim invoice divides a large project into scheduled payments at agreed milestones. Instead of waiting until project completion to invoice, the seller collects payment as work progresses. This protects cash flow on long projects and reduces the financial risk if the relationship breaks down mid-project.

➡️ When to use an Interim Invoice

For multi-month projects: construction, software development, large marketing campaigns, and any engagement where the full cost is too large to collect at completion.

Example: A construction firm takes on a 12-month building contract worth $240,000. The contract specifies four interim invoices at the completion of each quarter, plus a final invoice at handover.

A timesheet invoice combines a timesheet and an invoice into a single document. The total charge is calculated from the hours logged, multiplied by the agreed hourly rate. It is common for contractors, consultants, and any service business that bills by the hour.

➡️ When to use a Timesheet Invoice

When charging for billable hours rather than a fixed project fee.

Example: A consultant charges $150 per hour and works 20 hours on a client project. They issue a timesheet invoice showing the hours broken down by date and task, with a total of $3,000 due.

A recurring invoice is sent to the same client for the same amount at a regular interval. The most common use is subscription billing: internet services, software licences, cleaning contracts, and maintenance agreements. Most invoicing software can automate recurring invoices so they generate and send without manual input.

➡️ When to use a Recurring Invoice

When the same invoice repeats on a fixed schedule. If your invoicing software supports automation, configure these as auto-sends — removing the manual step eliminates the risk of a missed billing cycle.

Example: A cleaning company invoices a client every Friday for a weekly cleaning package at the same rate. The invoice is identical each week except for the date and invoice number.

A credit invoice, also called a credit note or credit memo, is issued when money is owed back to the buyer. This happens after a returned item, a damaged delivery, an overcharge, or a discount applied after the original invoice was paid. Credit invoices always show a negative total amount.

➡️ When to use a Credit Invoice

When a transaction needs to be partially or fully reversed. Issue the credit invoice before processing any refund — it creates a paper trail that protects both parties if the original invoice is later disputed.

Example: A client received machine parts, but three arrived damaged. The supplier issues a credit invoice for $300 (the value of the three parts), which the buyer can apply against their next invoice.

A debit invoice, also called a debit memo or debit note, is issued to increase the amount the buyer owes. This is used when additional charges arise after the original invoice was sent: a scope change, a miscalculation, or the addition of delivery fees not included initially.

➡️ When to use a Debit Invoice

When an additional charge needs to be added to an existing invoice.

Example: A supplier already invoiced a client for 60 machine parts. During shipping, the client requested an urgent courier upgrade. The supplier issues a debit invoice for the additional $80 delivery surcharge.

A mixed invoice combines a credit adjustment and a debit adjustment into a single document. The net total can be positive (the buyer owes money) or negative (a credit is owed). It is used to consolidate multiple adjustments rather than issuing separate credit and debit notes.

➡️ When to use a Mixed Invoice

When you have simultaneous credits and debits to apply to the same client account.

Example: A supplier issues a credit of $200 for damaged goods and a debit of $80 for additional delivery costs. Rather than two separate documents, they issue a mixed invoice showing a net debit of $120 due.

A final invoice is issued at the end of a project to settle the remaining balance. It deducts any amounts already paid through retainer or interim invoices and shows the outstanding amount due. A final invoice signals that the work is complete and the billing relationship is formally closed.

➡️ When to use a final invoice

When the last deliverable or milestone has been signed off. Include a summary of previous payments on the invoice itself so the client can verify the outstanding balance without cross-referencing earlier invoices.

Example: A construction firm completes a $240,000 project. Three interim invoices totalling $180,000 have already been paid. The final invoice is issued for the remaining $60,000.

An electronic invoice (e-invoice) is an umbrella term for any invoice that is created, sent, received, and processed digitally. The format varies by workflow:

• PDF invoice sent by email — the most common form for small businesses; easy to produce but still processed manually by the recipient
• Invoicing software — platforms that generate, deliver, and track invoices within a single system, with payment status updated automatically
• Structured data formats (e.g. XML) — used in automated B2B workflows where the invoice is read and processed by the buyer's system without any manual input

E-invoicing is growing fast. For small businesses, adopting it through software eliminates manual data entry and helps streamline payment processing.

⚠️ Regulatory note: The US currently has no federal B2B e-invoicing mandate — adoption is voluntary. However, it is accelerating among businesses with high invoice volumes and those trading with international partners who operate under mandate regimes in countries such as France, Italy, and Germany.

➡️ When to use an electronic invoice

Any time you want faster invoice delivery, automated processing, or a paperless workflow. For businesses with high invoice volumes, e-invoicing reduces errors and speeds up accounts receivable cycles.

Cash flow problems are rarely caused by a lack of revenue. They are caused by a gap between when money is earned and when it arrives in your account. How you structure your payment terms — and how quickly you follow up — is the primary lever for closing that gap.

Payment terms determine when you get paid.

Shorter terms are often accepted without pushback if you ask for them. Most businesses default to Net 30 without testing whether their clients would accept less.

Late payments compound the problem. When an invoice goes unpaid past its due date, it moves from accounts receivable into a grey zone: technically owed, but increasingly unlikely to be collected. Once an invoice reaches 90 days past due, the recovery rate drops to around 18%. The longer it ages, the less likely you are to see it paid.

Invoicing without a system is how accounts receivable problems start. If you are still building invoices manually from scratch each time, our guide on how to make an invoice covers the setup. 

These seven practices will cut the time you spend chasing payments and reduce the number of invoices that go unpaid.

Invoicing software automates the most error-prone parts of the process: generating invoice numbers, calculating totals and taxes, and tracking payment status. It keeps all your invoices in one place, which makes reconciliation and financial reporting straightforward — and it reduces human error. A miscalculation on an invoice creates a dispute and delays payment.

Well-known options include PayPal Invoice, Invoice2go, Zoho Invoice, and QuickBooks. For businesses that want their invoicing linked to their business account and payment flows, Statrys offers built-in invoicing at no additional cost.

🔍 Tip: See the 5 best free invoicing software in the market.

An invoice that is hard to read is more likely to be delayed. Put the most important information at the top: your business name, the client name, the total amount due, and the payment due date. Line items should be clearly labelled with enough description that the client knows exactly what they are paying for.

Include all accepted payment methods clearly on every invoice: bank transfer details (routing number and account number for US payments), any card payment links, and whether you accept international payments. For clients outside the US, specify the accepted currency and whether FX costs are the buyer's responsibility.

Use the same template for every invoice. Clients who receive invoices in different formats each time are more likely to misplace or misprocess them.

Common US payment methods

• Domestic bank transfer (ACH — Automated Clearing House) or wire
• Credit or debit card
• PayPal or other payment links
• Cheque

💡 For international clients, a multi-currency business account simplifies receipt of cross-border payments and removes the friction of currency conversion for the buyer.

Every invoice needs a unique invoice number — this is what you and your client will use to track, reference, and reconcile each transaction. 

📌 Suggested format: INV-[Year]-[Sequential Number] — e.g. INV-2026-0042

Use letters and numbers rather than digits alone: easier to reference in conversation and harder to misread.

Send your invoice the moment the work is done or the goods are shipped. Waiting a week to invoice a client puts you a week further back in the payment queue. 

Clients pay invoices in batches: if your invoice arrives after the payment run has been processed, you wait for the next one.

Net 30 is the most common standard in the US, but many businesses successfully use Net 15 for smaller invoices or with clients who have a good payment history. 

Net 60 is appropriate for large enterprise clients with their own payment cycles, but push back if shorter terms are possible.

A reminder sent before the due date is professional, not pushy. Most clients appreciate the reminder and pay on time as a result. A reminder sent after the due date should be factual: state the invoice number, the amount, and the new total if late fees apply.

Recommended schedule:

• 7 days before due date: a short, polite reminder with the invoice attached
• 1 day before due date: a brief heads-up that payment is due tomorrow
• Day of overdue: notification that payment has not been received, with the invoice reference and late fee information if applicable
• 7 days overdue: a firmer follow-up. Cc the accounts department if you know the contact.

If you are using invoicing software, set these reminders up once as automated emails.

Store your invoices in a cloud service so they are accessible, searchable, and recoverable if a device fails.

⚠️ Compliance note: The IRS requires US businesses to keep tax records for a minimum of three years, and longer in some cases involving employment taxes or amended returns.

Cloud storage options: Google Drive, Dropbox, OneDrive, and Apple iCloud all provide secure, redundant storage. Most invoicing software archives your invoices automatically, which is the easiest solution if you are already using a platform.