PP sellers can have 2 kinds of accounts. First type may receive only cash, from bank accounts or other PP accounts. This one charges the seller no credit card fee. The other type of account can receive payments from credit cards, debit cards, directly from banks, and from other PP accounts. It charges a fee irrespective of whether you pay that account with a credit card or not.
So, some sellers have 2 separate accounts - one to receive cash, and one to receive credit cards. They have to pay a fee on the latter one, so they often pass that fee on to the buyer by saying "add X%" if paying by credit card.
So, if you are paying with cash to the first kind of account, you should pay no extra fee. If you are paying with cash to the second kind of account, the seller may seek a fee from you even though you arent paying with a cc, because they are charged a fee on any kind of transaction to that account.
Sorta confusing.
peace