usps shipping flat rate international is a service offered by the United States Postal Service that provides flat-rate shipping for international parcels. With flat-rate pricing, customers pay a fixed fee based on the size of the box, regardless of the weight or destination. USPS Flat Rate International services include Priority Mail International and Priority Mail Express International. This service is particularly useful for shipping heavier items at a predictable cost. USPS offers various flat-rate boxes and envelopes to suit different types of shipments.