Congrats, indeed! Ooh, I am so hyped to buy one of these.
Good timing! Literally two days ago I took apart an old PS2 keyboard (a crappy early 2000s dell one) and was figuring out its key matrix. I was planning to get a breadboard and make a janky setup where you plug in leads from the two contacts of a dummy controller's individual buttons to the different lines of the matrix. I had picked out the controller for it, but I haven't begun hacking it up nor have I bought the breadboard and leads I'd need. You might've saved me a lot of hassle with this perfect timing. There's a part of me that still kind of wants to do it, but I'll put this on the backburner for now.
A suggestion, if I may: If you're planning to use a bubble mailer (I think that's what you're referring to), find out if they're free. The envelopes themselves, I mean. If so, I'd suggest double or triple bagging them for extra padding. I don't know if it is due to covid, or what, but over the last year I've had a lot of packages come to me from overseas that look like they were thrown down several sets of stairs with a spike trap at the bottom. And not just from China, but one from Australia, one from Scotland, a couple from Russia. Nearly everything shows up looking like it's been in a war. Hrm. Maybe it's USPS that's doing it, now that I'm thinking about it. Maybe they arrive here fine, but then USPS just throws them around when they get bored or something.
Also, I'm not familiar with Skrill (I'm not picky, I'll go with whatever you choose), but you could always use Tindie. Seems like just about everyone who is making indie electronics projects sells them on there these days.