Your engagement letter says net 28. The cash, on average, arrives on day 67. That is the gap a 2025 study of US B2B invoicing found across professional services: firms extend four-week terms, wait more than nine weeks, and see 39% of their invoices paid late outright. In the meantime you cover payroll, rent, and software for work you already delivered - which is another way of saying you run a small, interest-free bank on the side.
Most firms treat this like weather: unpleasant, universal, nothing to be done. It is not weather. It is a design choice about when the money conversation happens. Firms that move that conversation to the signature, instead of 28 days after delivery, collect in hours instead of months. Here is the numbers-first case, the exact mechanics, and a scripted sequence for the aged invoices already on your books.
What a 67-day wait actually costs
Days sales outstanding is the polite phrase. The impolite version: a firm billing $60,000 a month at a 67-day DSO is floating about $134,000 of delivered work at any given moment. Payroll went out. The rent cleared. The cash for the work that funded both is sitting in someone else's operating account.
The write-off number is worse than it sounds. Firms write off roughly 4% of receivables as bad debt - on $720,000 of annual billings, that is about $29,000 a year that simply evaporates. And the aging curve is steep: collections data consistently shows that an invoice crossing 90 days past due is worth about half its face value. An unpaid invoice is not a stable asset waiting to be collected. It is a melting one.
Most late payments are a process failure
The instinct is to read a late payment as a statement about the client: cash-strapped, disorganized, or stalling on purpose. The data points somewhere less dramatic. Surveys of B2B payment behavior attribute roughly 61% of late payments to administrative friction - problems on the sending side, not the paying side:
- Late delivery. The invoice went out days or weeks after the work did, so the clock started late and the memory of the value had already faded.
- Wrong contact. It landed in the inbox of someone who cannot approve it and sat there until somebody thought to ask.
- No payment path. A PDF with bank details instead of a pay link turns paying into a task, and tasks get deferred.
- Disputable details. A missing PO number or an unexplained line item hands accounts payable a legitimate reason to park it.
Every item on that list is an automation problem, not a persuasion problem. Invoices that generate the moment work is approved, route to the billing contact on file, and carry a one-click card or ACH pay link erase the 61% without a single uncomfortable phone call. But automation only speeds up the old model. The bigger win is retiring the model.
Put the payment authorization in the engagement letter
The highest-leverage change is one clause. At the moment the client signs - the moment their goodwill and motivation peak - capture a card or ACH authorization and store it on file. From then on, getting paid is something that happens on a schedule, not something you request and hope for.
- Recurring engagements. Monthly retainers, bookkeeping, and advisory fees charge automatically to the stored method on the first business day of each month. The invoice becomes a receipt, and the chase stops existing because there is nothing to chase.
- Project work. A deposit of 25-50% is charged at signature, with the remainder auto-charged on delivery or at named milestones spelled out in the letter.
- One-off work. Full payment, or at minimum a stored authorization, is captured when the work is booked - before it enters the queue, not after it leaves it.
The letter language is short: "Fees are charged automatically to the payment method on file on the first business day of each month," or for projects, "the remaining balance is charged upon delivery of the deliverables named above." One caveat: this is operational guidance, not legal advice. Card-on-file and surcharge rules vary by state and card network, so have your attorney read the authorization clause before it goes into the template.
The awkward conversation happens once, not twelve times a year
The objection is always the same: clients will balk. In practice, your average client already pays for insurance, payroll software, streaming, and their phone by autopay, and has for years. What clients find strange is not automatic billing. What they find strange is a professional firm that mails a PDF and hopes.
At signing, the script is one sentence: "We take a card or bank authorization now and bill automatically on the schedule in the letter - you get a receipt each time, and you never have to process an invoice from us." Said at signature, it is a convenience. Said to a client who already owes you money, it is a collections call. That is the whole trick: move the money conversation to the one moment it is easy.
Clean up the legacy book with three emails and one text
New terms fix new engagements. For the aged invoices already on the books, run a fixed sequence: firm but warm, every message carrying a live pay link, nothing improvised. Automate the sends so the cadence never depends on memory.
- Day 1 past due - email. "Hi [first name], a quick note that invoice [number] for [amount] came due on [date]. Here is the payment link in case it slipped through: [link]. If anything on it looks off, reply and we will fix it today."
- Day 10 - email. "Following up on invoice [number] for [amount], now 10 days past due. You can settle it in under a minute here: [link]. If timing is the issue, tell us - we would rather set up a payment plan than keep sending reminders."
- Day 20 - text message. "Hi [first name], this is [your name] at [firm]. Invoice [number] ([amount]) is 20 days past due - you can pay in one tap here: [link]. Thank you."
- Day 30 - email. "Invoice [number] for [amount] is now 30 days past due. Per our engagement terms, we will need to pause active work on [date, 15 days out] unless the balance is settled or a plan is in place. We value the work we do together and would much rather keep going: [link]."
Then enforce the pause. Work stops at 45 days past due and resumes when the balance clears and a payment authorization goes on file - the same clause new clients sign. Apply it to every client with no partner-discretion exceptions, because the moment one client learns the deadline is soft, every deadline is. Announced at day 30 and applied consistently, most balances clear before the pause ever triggers.
What to change this week
Three moves. First, add the authorization clause to your engagement letter template so every new signature arrives with a stored payment method attached. Second, load the four-message sequence into your invoicing system and switch it on for everything 1-90 days past due. Third, triage everything older than 90 days honestly: it is worth about half of face value now, so settle it at a discount, put it on a plan, or write it off and take the attention back.
Extending credit is a service banks charge real interest for. You have been giving it away with every letter that reads net 28 and means net 67. Stop being the bank. Be the firm that gets paid at the signature.
Frequently asked questions
Attach the change to a natural moment - the annual renewal, a scope change, or a rate adjustment - and frame it as how the firm now bills all clients. Offer ACH if card fees are a concern. Most clients agree without pushback, and the few who refuse are usually your slowest payers already, which is information worth having.
It is increasingly standard across accounting, advisory, bookkeeping, and agency work, and clients rarely question it when it is presented at signature. You do need clear written authorization language, and rules on stored credentials and card surcharges vary by state and network, so have your attorney review the clause before rollout.
Against net-28 terms, anything under about 35 days is healthy, while the professional services average runs near 67. Firms that charge at signing or on delivery push effective DSO toward zero on that work, which is the real target: not collecting faster, but removing collection as a step.
Set a fixed threshold - 45 days past due is common - announce it in writing at day 30, and apply it uniformly to every client. A stop-work rule enforced occasionally trains clients to test it; one enforced consistently almost never has to be used, because balances clear once the date is real.