The short answer
Almost every payment problem an agency has was decided in the first week, before anyone was thinking about money. Payment setup belongs in onboarding, alongside access and introductions, rather than in billing.
Ask the admin questions while the client is still in signing mode.
Start the slow items on day 1, because supplier setup and tax forms take weeks.
Write the answers down somewhere other than 1 person's inbox.
Send the first invoice early and small, to test the route before it matters.
Fix anything the test invoice exposes while the relationship is new.
The window is short and it closes quietly. In week 1 a new client will answer 6 administrative questions in a single reply. By week 4 the same questions read as an interruption to interesting work, and by week 10 they read as a complaint.
Why week 1 is the only easy time
At the start of an engagement everyone expects admin. Contracts are being signed, accounts are being shared, people are being introduced. A request for billing details sits comfortably in that stream and nobody thinks about it twice.
Once delivery starts, the client's attention moves to the work, which is where you want it. Administrative questions then compete with that attention and lose. The questions do not get easier later; they get more expensive, because by then an invoice is usually already stuck.
There is a second reason, less obvious. In week 1 you have a reason to ask that implies nothing: you are setting up. In week 10, asking who approves invoices implies that something has gone wrong, and the client hears it that way.
The 8 things to establish
What to establish | Who answers it | What it prevents |
|---|---|---|
Which legal entity to invoice | Your contact, after checking | An invoice rejected for the wrong company name |
Where invoices should be sent | Accounts payable | An invoice sitting in a project inbox |
Whether a purchase order is needed | Finance, not your contact | An invoice that never enters the queue |
Who approves, and who signs off above them | Your contact | An approval stuck behind 1 person on leave |
When the payment runs happen | Accounts payable | Missing a run by 2 days and waiting a month |
Supplier setup and tax forms | Procurement | Weeks of delay before anything can be paid |
Payment method and currency | Both sides | A transfer that arrives short or late |
Terms, and what the clock starts from | You, in the contract | An argument about whether an invoice is overdue |
Most of these are covered individually elsewhere, including the 6 questions worth settling before you start. What matters here is doing them together, once, as a single piece of setup rather than as 8 separate discoveries made over 3 months.
The third row is the one agencies skip most. Your contact will often say no purchase order is needed, in good faith, because the requirement lives in a finance policy they have never read. Above a few thousand dollars, ask them to confirm with finance, for the reasons in the piece on PO numbers.
The email that collects it in 1 go
Send it when the contract is signed, before the kickoff call. 1 message, specific questions, and an offer to do your half first:
Subject: Billing setup for [project]
Hi [Name],
Before we start, I'd like to get the billing admin out of the way so the first invoice isn't held up.
From us, you'll need: our completed [W-9 / W-8BEN-E], our bank details and our registered entity details. I can send all 3 today.
From you I need 5 things, and whoever handles supplier setup can usually answer them in a line each:
1. Which legal entity should the invoice be addressed to?
2. Where should invoices be sent?
3. Is a purchase order required, and if so who raises it?
4. Who approves invoices, and is there a second approver above them?
5. When do your payment runs happen?
If it's easier, point me at the right person and I'll ask them directly.
Thanks,
[Name]
The last line does more work than it looks. Plenty of contacts have no idea what their payment runs are, and offering to go directly to finance lets them hand it over without having to admit that. You get better answers and they get less work.
Start the slow 2 on day 1
Supplier setup and tax forms are the items that run on someone else's timetable, and they are the usual reason a first invoice is late even when everything else went well.
Supplier onboarding often means a procurement platform, bank detail verification and sometimes insurance certificates. Bank verification in particular is deliberately slow, because it is the step most exposed to fraud. Weeks, not days.
The tax form has a sharper edge in the US. Where a payee does not furnish a correct taxpayer identification number, the payer is required to apply backup withholding, which the IRS sets at 24%. A quarter of your invoice held back over a form you could have returned in week 1 is the most avoidable loss on this list. The cross-border version, including which W-8 applies if you are outside the US, is in getting paid by a client in another country.
Send a small invoice early
The most useful thing you can do in the first month is put a real invoice through the route before the amount matters. A deposit works perfectly for this, which is 1 more argument for taking one.
A test invoice tells you things no checklist will: whether the address works, whether the entity name is right, whether the purchase order covers it, whether anybody acknowledges receipt, and how long the client actually takes rather than what their terms say. Discovering all of that on a $2,000 deposit is cheap. Discovering it on a $18,000 final invoice is not.
Where the deposit is paid without friction, note how many days it took and use that figure in your forecast rather than the contractual terms. That difference is the gap a cash flow forecast has to fund.
Write it down somewhere shared
The answers are worth nothing if they live in the thread where they were given. Record them against the client, in whatever system you already use, with the date and the name of whoever confirmed each one.
Legal entity, registered address, and the invoice address.
Purchase order requirement, the number, and what it covers.
Approver, second approver, and the accounts payable contact.
Payment run dates, terms, and the terms start point.
Payment method, currency, and who bears transfer charges.
The date supplier setup completed, and the portal if there is one.
Sending invoices from a shared billing address helps here, because the thread itself then sits somewhere the whole team can find. The common failure is a founder who holds all of this in their head and their sent folder, which works until they are away in the week an invoice sticks.
When they will not answer
Some clients ignore the setup email. That is information rather than an obstacle, and it is worth reading.
Ask once more, then go directly to accounts payable, whose job this is and who will usually answer in a line. Where neither responds, note it: a client who will not tell you how to invoice them in week 1 is unlikely to be quick about paying in week 12, and it is a reasonable moment to look again at what their public payment record shows.
It is also a reason to keep the exposure small early. Stage the billing so the first stages are modest, as in milestone billing, rather than discovering a problem on a large balance.
The 10-minute version
For a small client with no procurement function, most of this collapses. Ask 3 things and move on: which entity to invoice, where to send it, and who approves. Agree the terms in the contract, take a deposit, and send the first invoice promptly.
The checklist scales with the client's size, and the point of knowing the full version is recognising which rows apply. A 6-person studio has no payment runs. A listed company has all 8 rows and a portal.
FAQ
When should I ask a new client about payment admin?
In the first week, while the contract is being signed and administrative questions are expected. The same questions in week 10 imply something has gone wrong, and by then an invoice is usually already stuck.
What should I ask a new client before invoicing?
Which legal entity to invoice, where invoices go, whether a purchase order is required, who approves, and when payment runs happen. Then settle terms, method and currency in the contract rather than by email.
Why does supplier setup take so long?
Because bank detail verification is deliberately slow, being the step most exposed to fraud, and procurement platforms add their own queue. Treat it as weeks and start it the day the work is agreed.
What happens if I don't return a W-9?
For a US payee, the payer is required to apply backup withholding at 24%. It is entirely avoidable and it is the single most expensive item to leave until later.
Should I send a test invoice?
Yes, and a deposit is the natural one. It checks the address, the entity, the purchase order and the real payment speed while the amount is small enough that a problem costs you nothing.
What if the client never answers the setup questions?
Ask again, then approach accounts payable directly. Persistent silence about how to invoice is a signal worth acting on, so keep the early stages small while you find out more.
Not legal or tax advice. Tax forms, withholding and supplier requirements depend on your country and your client's. Confirm your own position with a qualified adviser.