Well, first of all I must say that there is something very strange in this forum. I very often see the word "donation" used, when it means "purchase" in the context used. (Example: "To get the source code you must donate €10 to
[email protected]") It's so commonly used like this that I can't imagine it's because of the language barrier, it's just some weird forum culture. But lets set that aside for the moment and rephrase your question to this:
”A person in the forum post a request [for support, solution, code, etc] and say they are willing to pay $10 for it. If you tell them you are able to provide it to them and they approve, and you then deliver - and they don't pay, what should you do?”
I would start with the assumption that they aren't evil or scammers. Give them the benefit of a doubt, message systems fail all the time. And life does get in the way sometimes.
So I would start by asking them a couple of times via private messages or emails for payment.
1. If they pay - all is good
2. If they don't respond to messages at all: Post something like this in the thread where they requested the help:
”I have been trying to reach you several times on private messages and emails, to handle the payment for the agreed solution. There might be some problem with the forum or the email server because I never get a response. So I'll ask here in the forum instead that you pay me per our agreement. Please contact me at ... if you want to discuss something, otherwise I look forward to your payment as soon as possible. Thanks."
3. If they respond but refuse to pay: Post something like this in the thread where they requested the help:
”We have communicated several times over private messages and emails, to handle the payment for the agreed solution. In your post #1 you clearly stated what you needed, and what you were willing to pay. In my post #2 I said I could do it, to which you agreed in post #3. Two weeks later I was done and did send you the solution to the email address you provided in a private message. When I now request payment you refuse to pay me, even though I provided exactly what you requested. You have been given the chance to come up with feedback or adjustments required in my delivery, but you have not done so, so I can only assume that my delivery was in fact perfect. Even so, you refuse to pay me. The forum is hereby warned. I am willing to edit this post if you pay me as soon as possible and send payment information in a private message."
4. If they agree to pay but payment never shows up even though you have talked about it in private several times: Post something like this in the thread where they requested the help:
"You wanted X for payment $10, and I offered to help, which you agreed. On January 10th of 2022 I send you X to your email, as you instructed me in a private message. You did not pay me upon delivery, and I have since reminded you and asked for mayment three times in private. Each time you come up with explanation for why the payment is delayed. I believe I have been more than fair and understanding of your issues with regard to payment. At this point, however, I must assume that you have no intention to actually pay what you owe me. So I post this in the forum as a last try to get the payment that I am owed. The forum is hereby warned. I am willing to edit this post if you pay me as soon as possible and send payment information in a private message."
(Yeah, these example messages are bit long. Sorry, I didn't have time to write a short text.)