I've definitely noticed the different banks/rates issue myself. I don't know of any alternative sites unfortunately, but I also wanted to say some sellers helped me figure out that a lot of the issue is more so the world economy prices fluctuating a ton right now. Especially if it's through Boosty or a similar site where the money isn't taken from you formally until the seller accepts your money (or that's my understanding at least), there can be a lag that further affects the price. Sadly, I don't know what can be done to help this, if anything, but I just wanted to say you're not alone in this issue.

Also in regards to adopt prices, I have noticed that as well, but I know at least as of where I live, the cost of living went up this year by $35-50, depending on where you live exactly (amount in USD)!! So I think things like that can really drastically hit artists hard, too, and drive prices up.

I hope you can find info to help you. Also, your art is lovely, and please try not to take things too harshly on yourself. I've seen a lot of creators blame themselves for monetary issues when it's really more to do with various outside forces across the world, especially since the pandemic still prevents so much steady work to be possible.