Ravi

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

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.