price usps flat rate boxes is a standardized pricing structure offered by the United States Postal Service. The price for USPS flat rate boxes depends on the size of the box and the service used. For example, the USPS Medium Flat Rate Box typically costs around $15.50 for Priority Mail, while the Large Flat Rate Box is priced at around $21.50. These flat rate options are convenient because they allow you to ship packages without needing to weigh or measure them, making them ideal for heavier items that would otherwise cost more with regular pricing.