Most large supermarkets (e.g., Publix, Kroger) sell stamps at the checkout or customer service desk and accept credit cards.

You can use a credit card on the official USPS Postal Store or via third-party sites like Amazon and Stamps.com . Accepted Payment Methods at USPS

The US Postal Service officially accepts the following credit cards: MasterCard American Express Digital Wallets (Apple Pay, Google Pay) Payment Acceptance Policies - About USPS Home

You can use a credit card at full-service retail counters and USPS self-service kiosks.