Avoiding Common Invoice Errors for Timely Payments

Invoice checklist before you send

Use this checklist before you send each invoice. Each item is a common reason for a client to return an invoice or to pay it late. Two terms appear often below. Accounts payable (AP) is the team that pays supplier invoices. A purchase order (PO) is a client document that approves a purchase.

Client details

• The legal name of the client, as it appears in the contract
• The billing address that the AP team uses, which can differ from the head office address
• The name or inbox that receives invoices
• The PO number in the exact format of the client, if the client uses POs

Number and dates

• A unique invoice number that follows your sequence and is never used again
• The invoice date
• The date or the period of the work or the delivery
• The due date or the payment terms from the contract

Description and amounts

• A description of each deliverable with the project name, the contract or statement of work (SOW) section, and the dates
• A correct line total for each line (quantity multiplied by rate)
• A subtotal that equals the sum of the lines
• Discounts and tax in the order that your tax rules require
• The correct tax rate for the place where the tax applies
• The total, with the currency as a three-letter ISO code, for example USD or EUR

Payment details

• All payment methods that you accept
• For bank transfers: bank name, account number, routing number, and the SWIFT code for international payments
• For card payments: a payment link that works
• For payment by check: the full mailing address
• Late fee terms, if your contract includes them

Your details

• Your business name and address
• Your phone number or email address for billing questions
• Your tax or VAT number, if your tax rules require it on invoices

Why invoices come back

Error	What happens	How to prevent it
Wrong client name or address	The AP system cannot match the invoice to a supplier record	Get the legal name, billing address and invoice inbox before the first invoice
Missing or wrong PO number	Many AP teams do not process an invoice without a PO match	Ask if the client needs a PO before you start the work
Duplicate or skipped invoice number	AP systems can reject a number that they saw before	Use one sequence for all clients and keep voided numbers in your records
Vague description	The approver cannot match the charge to the work	Name the deliverables, the SOW section and the dates
Math error	The client asks for a corrected invoice	Use invoicing software or a spreadsheet with formulas
Wrong tax rate	The invoice stops while someone finds the correct rate	Look up the rule for the location where tax applies
Incomplete payment details	The client does not know how to pay	List full details for each payment method
Wrong recipient	The invoice waits in the inbox of your project contact	Send it to the AP inbox or the person that the client named
Unclear currency	The client pays in the wrong currency	Write the ISO code and use the currency from the contract
Late invoice	Budgets closed or approvers changed jobs	Send the invoice soon after each delivery or milestone

A description such as “Design services, March” gives the approver no way to match the charge. Compare it with this description: “Social media graphics for the spring launch (12 posts, 6 ads) per SOW section 2.3, delivered March 15.” The second description names the work, the contract and the date.

Workflow from finished work to sent invoice

1. Finish the deliverable or reach the milestone.
2. Make sure that the client details, the PO number and the invoice inbox are current.
3. Go through the checklist above.
4. Send the invoice within a few days of the delivery.
5. A few days later, make sure that the client received the invoice.

Clients do not always tell suppliers when the AP contact or the PO changes. At the start of each new project, ask the client if the billing details are still correct.

If you invoice in the UK

GOV.UK lists what an invoice must include. The list has a unique identification number, your company name, address and contact details, and the name and address of the customer. It also has a clear description, the supply date, the invoice date, the amounts, any VAT amount and the total owed. A limited company must use its full name as it appears on its certificate of incorporation. If you and your customer are both VAT registered, you must use a VAT invoice.

If you and the customer did not agree on a payment date, UK law sets one. A business payment is late 30 days after the customer gets the invoice. If you deliver the goods or the service later than the invoice, the 30 days start at the delivery. Other countries have their own rules. If you charge VAT or GST, make sure that your invoices meet the rules of that tax authority.

Use this checklist with Claude or ChatGPT

Claude or ChatGPT can review a draft invoice against this checklist before you send it. Paste the checklist into a new chat. Then add the draft invoice and the PO or the contract terms, as text or a PDF. Ask the AI to list each missing or wrong item. For example:

Below is my invoice checklist, a draft invoice and the client's
purchase order. Go through the checklist item by item and mark
each one OK, MISSING or WRONG, with the line it refers to.
Recalculate every line total, the subtotal, tax and total, and
show your arithmetic. Flag any difference from the purchase order.

Before you send the invoice, read these cautions:

• Recalculate the totals yourself. The AI can make arithmetic mistakes or misread a number in a PDF.
• Remove bank account numbers before you upload the invoice. Also read the data settings of your workspace and the confidentiality terms of your client contract.
• Do not rely on the AI for tax rates. Get the rate from the tax authority or from your accountant.

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