The definition of a donation is that you don't have to give it.

If you have to pay then it's either a purchase or a tax.

But you know this of course.

I do know that, but I also know how donations can become an expectation.

Also, it's worth noting in the context of this thread, that people can use AI inference for free on many services, with payment only need for higher usage, and even then, if you don't care about expectations or inconvenience, it's trivial to abuse the free tier.