Here is the opening line of almost every payment reminder I have ever written, and probably every one you have written too.
"So sorry to bother you again, but I just wanted to check on the invoice from last month."
Read it back slowly. In eleven words I have apologised for existing, called my own invoice a bother, described chasing money I am owed as "just checking," and softened "last month" until it sounds optional.
The client has done nothing wrong in that sentence. I have.
What the apology actually does
An apology assigns a role. When you open with sorry, you are the one who has caused an inconvenience, and the person reading is the one being inconvenienced. That framing then sits underneath the entire message, and the client absorbs it without noticing, because nobody reads an email consciously enough to object to its architecture.
So they file it as a low priority request from someone who is slightly embarrassed to be asking. Which is precisely how you described yourself.
The uncomfortable part is that this is not a confidence problem. I know plenty of freelancers who negotiate hard, hold their rates, and still open payment reminders like they are asking for a favour. It is a reflex, learned from every polite email we have ever seen, and reflexes do not respond to pep talks.
They respond to structure.
Open with the two facts that are not in dispute
The invoice number and the original due date.
"Invoice 0042 was due on 18 June."
That is it. No adjectives, nothing to disagree with, nothing that reads as emotional. You have not accused anyone of anything, you have stated a document reference and a date, and both of those are simply true. The client's own accounting system would say the same thing in the same tone.
What this does psychologically is worth understanding. A sentence with feeling in it invites a reply with feeling in it, and feelings are negotiable. A sentence with a date in it invites a reply about the date. You want the conversation to be about the date.
Then keep the warmth, but move it
I am not telling you to be cold. The relationship is the whole business. I am telling you the warmth belongs at the end, not the front.
The structure I use now runs in four parts, and it fits inside six lines.
The fact. Invoice number, amount, due date.
The ask, with a date attached. Not "at your earliest convenience," which is not a date and never has been. Something like "could you confirm when this is going out." Notice that asks for information, not for money, and information is far easier for a client to hand over than a payment. Very often the honest answer is that it is stuck with someone in finance, and now you know that.
The bridge. One line that keeps the project moving, because you are still working together and neither of you wants this to become the entire relationship.
The warmth. Sign off the way you normally would with this person.
Here is the whole thing.
"Hi Sarah, invoice 0042 for 180,000 was due on 18 June. Could you let me know when it is scheduled to go out? Happy to resend it if it needs to go to a different address. The homepage revisions are with you either way, no rush on those. Thanks, Depender."
Twelve seconds to read. Nothing in it can be argued with. No apology anywhere.
Two smaller things I got wrong for years
"Just" is doing damage everywhere. Just checking, just wanted to ask, just following up. Every instance of it shrinks the sentence around it. Search your draft for the word and delete each one, then read it again. The message does not become rude. It becomes normal, which is the level you were aiming for and undershooting.
Do not stack reminders in the same thread forever. After the second one, reply-chain fatigue sets in and the client stops seeing it. Start a fresh email with the invoice number in the subject line. It reads as a new administrative item rather than a nag, which is exactly what you want it to be.
The thing nobody tells you
Different clients need different versions of this. A long-term client who has always paid on time and is clearly just buried needs a much lighter touch than one who has gone silent twice before. Same facts, different shape.
I used to rewrite these from scratch every time, badly, at eleven at night, with more apology in them each round. That is genuinely why I ended up building something to remember how each client needs to be spoken to. But the fix that mattered most was smaller than any tool.
Stop saying sorry for sending an invoice. You did the work.