I’m building a mobile wallet that works on both iOS and Android and relies on NFC cards as the primary payment token. The app must let users recharge their NFC card to use the card for payments in a closed environment. Core flow: a user registers, links an NFC card, tops up balance, then pays a merchant using their NFC card. The transaction should post instantly, deducting the right amount and updating the customer’s balance in real time. Acceptance is complete when an NFC card can fund, pay and show the updated balance without errors under normal use.