@Billy ********
well, technically you are right - you can merge several docs into 1 pdf. However, if there are no place in the e-visa system to upload accomodation details, then where are you planning on adding this pdf?
I suggest that you create an account on the Thai E-visa platform and start to add your details. That way you can decide if and where you should upload a pdf about your accomodation.
I understand that you want to plan ahead, but since you have a problem obtaining this document and the e-visa platform doesn't require it, my suggestion was, and still is, to apply with the documentation that are in fact asked for in the system and wait. They may or may not ask you to submit additional information. And they may or may not use the exact wording "proof of payment...", but there is a chance they might ask for it in a different way too, like in the NZ example "proof of accomodation, e.g. hotel booking..."
If it were me, I'd apply and wait to see what they ask for (if anything), but of course you do as you please. If you can come up with a solution on how to prove that you paid for accomodation that's great!