Interestingly enough, I spent about an hour today fiddling around with my site attempting to do exactly what you said. My business offers services not products to purchase. Therefore, I send invoices to my clients that could be any random amount depending on the work being done. I wanted a simple way for clients to go to my website to pay their invoices. I also tried turning the “donation” block into a “payment” block, but as you mentioned it wouldn't work without causing other issues and confusion. I ended up doing a live-chat with Squarespace asking if there was anything I could do to set up a simple “make a payment” process and was advised they don’t have a way to do that. I find it odd this isn’t possible to do as it seems something simple that many businesses would use and that would require very little for Squarespace to implement. They could tweek the “donation” wording around and a few other things and there you go!