Customer is forty days into net-30 and has stopped replying: how do you chase without torching the relationship?

I worked in accounts payable, so here is why this is happening, which changes what you should do.

Your contact is almost certainly not the person who pays you. He approves; someone else runs the payment. When an invoice sits, it is usually one of four things, and none of them is a decision not to pay you:

• It never reached accounts payable, it is in your contact's inbox, unforwarded
• It is in their system but unapproved, waiting on your contact to click something
• Something on it does not match - no purchase order number, a name that does not match their vendor record, the wrong entity
• It missed a payment run and is queued for the next one

That last one is why net-30 often means forty days in practice: many companies pay on fixed days, so an invoice that misses one waits for the next.

So the highest-value move is not a firmer email to your contact. It is contacting accounts payable directly and asking a specific question: "Can you confirm you have invoice 1043 dated the 2nd, and tell me which payment run it is in?" That is not escalation, it is routine, and AP deal with it all day. Ask your contact for the AP email, people give it willingly because it takes the problem off them.

Nine times in ten you find it was never entered, and it is paid within a fortnight.

The sequence I use, and I have never lost a client to it:

Day 0, invoice sent, terms on it, payment details on it.
Day 25, a short friendly note before it is due. "Just a heads-up that invoice 1043 falls due on the 2nd." This one does most of the work and it cannot possibly annoy anybody, because nothing is late yet.
Day 1 overdue - "This became due yesterday, could you confirm it is scheduled?" Neutral, immediate, no apology.
Day 7: same again, copying AP if you have them.
Day 14, a call. Not an email. This is where it moves.
Day 30, formal note referencing the terms and stating what happens next.

The thing I got wrong for years was treating a chase as a confrontation. It is admin. The friendliest possible framing, "has this got stuck somewhere?", is also the most effective, because it is usually true and it gives everyone a way to fix it without anyone being wrong.

Two things that matter more than the wording:

Be boringly consistent. A supplier who always chases on day one gets paid on time, because they become the invoice you deal with first. A supplier who chases erratically at day forty gets paid at day forty forever.

Pick up the phone at some point. Email is easy to leave. A two-minute call resolves what four emails did not, every time.

Consequences in the contract are what let the email be relaxed. You are referring to something agreed, not making a threat.

A year of being fine and then silence is almost always a process change on their side, not a decision about you.

For next time, the thing that lets you be relaxed in the email is having been firm in the contract. Put the consequences on paper up front and you never have to invent them mid-argument.

What is worth having written down before the work starts:

• Terms stated explicitly and repeated on every invoice, with a due date rather than "net 30"
• What happens when it is late, interest, or a suspension of service. Having it there means mentioning it is a reminder rather than a threat, which is the entire difference in tone
• Who to invoice, meaning the AP address, collected at signing rather than hunted for later
• Any purchase order or reference they require, because an invoice missing it will not be paid and often nobody tells you

For a subscription business the strongest lever is boring: payment before the period, not after. Card on file, or an annual invoice paid up front with a discount. Most of this problem disappears when you are not extending credit in the first place, and at your size you are effectively a lender who charges nothing.

On this specific invoice, though: the answer above is the right one. Find AP, ask a factual question, and expect it to be an administrative gap rather than a refusal. Save the contract talk for the renewal conversation.