A fee address is a designated address for receiving communications related to maintenance fee payments for patents. According to MPEP 2540, "Generally, notices, receipts, and other communications relating to the payment of a maintenance fee will be directed to the correspondence address used during the prosecution of the application, unless a 'fee address' for the purpose of payment of the maintenance fee has been designated or a change in the correspondence address has been made." It's important to note that "A fee address must be an address associated with a Customer Number" as stated in 37 CFR 1.363(c). To learn more: fee address maintenance fees customer number patent communication