付费 is a separable verb (离合词) meaning 'to pay a fee' or 'to pay the cost'. The verb 付 and the noun 费 can be split when adding aspect markers (了, 过) or measure words: 付了费 ('paid the fee'), 付过费 ('have paid the fee'). It is commonly used in contexts like membership fees, service charges, or tolls, and is more formal than 交钱.