how much does it cost to ship priority mail depends on the weight and size of the package, as well as the delivery zone. For example, shipping a small flat-rate box with USPS Priority Mail could cost a flat rate, while larger packages are priced based on weight and destination. As of now, the cost for a USPS Priority Mail shipment can range from $7.95 for small flat-rate boxes to higher costs for larger, heavier packages. Rates are updated regularly, so checking the USPS website or using a shipping calculator can provide the most accurate pricing.