Yeah, it sucks. I have been carefully adding up as I go for my 3 sites I can sell at, and I always add on extra over the listing for the shipping and taxes when I am calculating my numbers, to make sure I don't go over $600--and that matches their sales report info. I think I have maybe $25 room on each of the sites so I am done to avoid going over $600 on any of them. I know that both Mercari and Ebay show the amounts for selling with the taxes and shipping, and Posh has at least the shipping because the numbers added up to what I expected from the other sites.
I had two more items on ebay and realized that I would go over so pulled them a while ago. Any other items I have been selling have been through more informal methods.
Ebay does require your bank info when you sell something, or at least it appears so.
So yep it adds up fast!