Get set up as a supplier before you start the work, get a purchase order number, and submit each invoice through the portal exactly as the company asks, because an invoice that doesn't match the order can be set aside without anyone telling you. Then chase the accounts payable team directly and ask which payment run your invoice is on.
In this article
The short version
- Large companies usually pay through accounts payable, not the person who hired you.
- Finish supplier registration and get a purchase order before you start the work.
- Invoices that don't match the PO can be rejected or left in a queue.
- Chase accounts payable and ask which payment run your invoice is on.
- Adfin can't submit invoices to portals, but it can send reminders to the accounts payable contact you set, send statements and match payments.
How a large company pays a supplier
The person who asked you to do the work usually has nothing to do with paying you. In a large company, your invoice goes through a separate accounts payable team, often through an online supplier portal, and it's paid on a fixed payment run.
Before any of that, you'll be set up as a supplier. Expect a form or portal registration asking for your company details, bank account, VAT number and sometimes insurance certificates. Until that's done, there may be nowhere for your invoice to go.
Then comes the purchase order. Most large buyers raise a PO before work starts, and their system checks your invoice against it, and often against a record that the goods or service were received. If the PO number, amount, description or company name on your invoice doesn't match, the invoice can be rejected or left in a queue. Some portals send a rejection notice, but you may only find out when you chase the payment.
Before you send the first invoice
It helps to treat the first invoice as part of the job and agree everything about it before you start. Accounts payable teams usually need the following.
- Finish supplier registration, and check your bank details have been verified on their side.
- Get the PO number in writing before work starts, and check the amount and description match your quote.
- Find out how invoices must arrive: through the portal, by email to a specific address, or both.
- Use the exact legal entity name on the PO, which may differ from the brand you dealt with.
- Quote the PO number, and any line or receipt reference, on every invoice.
- Confirm the payment terms and whether they run from the invoice date or the date the invoice is approved.
- Get a named contact in accounts payable, and ask how often they run payments.
Public sector buyers work in a similar way, with their own rules on payment terms and how invoices are submitted, so ask about both when you're set up.
When an approved invoice still isn't paid
Chase accounts payable directly, with the invoice number, PO number and amount in the first line of your email. Ask two plain questions: is the invoice approved, and which payment run is it on. If it's been rejected, ask for the reason in writing so you can resubmit.
It helps to know how the company usually pays. Large companies publish how quickly they pay suppliers, and you can look them up for free on GOV.UK.
If you're a small business and a larger company owes you, the Small Business Commissioner may be able to look into the dispute for free, once you've tried to sort it out directly. You also have the right to claim statutory interest on a late business invoice, at 8% above the Bank of England base rate.
What payment software can and can't do here
Adfin can't submit invoices to a customer's supplier portal, so you'll still upload them yourself in the format the portal asks for. What it can do is keep things moving once the invoice is in.
If you set the accounts payable team as that customer's contact, Adfin's AI Customer Agents can send them reminders at the intervals you choose, with a payment link and a statement of everything outstanding, so nothing relies on your project contact passing emails on. You can also give each customer their own bank details in your business's name, which you enter once during supplier registration. When their payment run sends the money, it's matched to that customer's invoices even if the remittance carries their reference rather than yours. And for overdue business invoices, Adfin can add statutory interest automatically, with the option to waive it for customers you'd rather not charge.
Common questions
Why was my invoice rejected by a supplier portal? The most common reasons are a missing or wrong PO number, an amount or description that doesn't match the order, or the wrong company name. Ask accounts payable for the reason in writing and resubmit.
Who should I chase for payment at a large company? The accounts payable team. The person who commissioned the work often can't see or speed up the payment, though they can help get an invoice approved.
How can I find out how quickly a large company pays? Large companies publish how quickly they pay suppliers, and you can search for them on GOV.UK.
Can Adfin upload my invoices to a customer's portal? No. You submit invoices to the portal yourself. Adfin can send reminders to the accounts payable contact you set for that customer and match the payment when it arrives.
What can I do if a large company keeps paying late? Chase in writing, claim statutory interest, and if you're a small business, see whether the Small Business Commissioner can help.
Do I need a purchase order to invoice a large company? Usually, yes. Many large buyers won't pay an invoice without a valid PO number, so ask for it before you start the work.
Sources
- Small Business Commissioner - A guide for effective invoicing (accurate as of October 2026) https://www.smallbusinesscommissioner.gov.uk/help-and-guidance/all-advice/a-guide-for-effective-invoicing/
- GOV.UK - Check when large businesses pay their suppliers (accurate as of October 2026) https://www.gov.uk/check-when-businesses-pay-invoices
- Small Business Commissioner - FAQs (accurate as of October 2026) https://www.smallbusinesscommissioner.gov.uk/faqs/
- GOV.UK - Late commercial payments: charging interest on commercial debt (accurate as of October 2026) https://www.gov.uk/late-commercial-payments-interest-debt-recovery/charging-interest-commercial-debt
