I think it's just a part of the transaction as the seller. If I remember correctly, it's against paypal rules to charge the fee to the buyer.
I know that was the rule when we started with pay pal long ago. Your account would be closed if Pay pal found out from the customer that you charged them the fees. Now that is going way back & I'm not sure if it changed.
You want to see a big Pay pal fees? Try an american express Credit card used thru pay pal. There was a flat rate fee & 5.5% as best I can figure on the transaction.