priority shipping cost is a key factor when deciding between different shipping options. Priority Mail, offered by USPS, is a popular choice for sending packages quickly within the U.S. The cost of Priority Mail depends on the weight, size, and destination of the package. For instance, flat-rate boxes allow customers to ship packages of any weight for a fixed price, starting around $8.45 for small boxes. For larger packages, the price increases, with larger flat-rate boxes costing around $21.10. International Priority Mail costs more depending on the destination country.