Cash,  NO credit card.  Maybe, JUST MAYBE, a debit card.

From the USPS web site;

Credit and Debit Cards

For customer convenience, we accept credit and debit cards for retail transactions at all post offices. Visa, MasterCard, Discover, and American Express credit cards can be used to purchase all postal products and services except CODs, meter settings, and money orders. Contact your local post office to find out which debit cards are currently accepted in your area.

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