If you're expecting a certain amount for an adopt/piece of artwork due to fees/taxes, then please just put what you have up for a fixed price that'll cover the cost.

Just had a thing w/an artist where i had budgeted to pay a certain amount because that's literally all I have in my bank at the moment, but they sent me a 'friends and family' request with an additional charge for taxes. Not only is this INCREDIBLY rude, but this is also against PayPal's ToS.

Just. Don't do it. And if it happens to you, please make yourself aware that people literally cannot do that.