Why was a processing fee added to my card payment?

Because your workspace passes the cost of accepting a card to the customer, and the card used was a credit card.

Turn it on or off under Marketplace, Payments, with the switch "Pass card processing costs to the customer". The fee is what the card costs to accept, never more, and it is added only when all four of these are true: the switch is on, there is a balance to pay, the card is a credit card, and the customer's state allows it.

Debit cards, prepaid cards and bank transfers never carry it, and neither does a payment from a customer with no billing state on file. A few states set their own limits: Connecticut, Massachusetts and Maine allow no fee at all; Colorado caps it at 2% of the payment and Minnesota at 5%.

The fee is recorded on the payment as its own figure and is never applied to the invoice, so it does not change what you have collected or what a customer still owes. The customer sees the fee and the new total before paying, and again on the receipt. In your accounting system it posts as recovered processing cost, not as sales.