USPS Priority Mail Flat Rate Boxes: Costs, Sizes, and Usage Guide

Key Takeaways Large Flat Rate Box: $24.75 retail (up to 70 lb) regardless of weight or distance. Small Flat Rate Box: $10.40 retail (up to 70 lb) regardless of weight or distance. Flat Rate Envelopes range from $9.85 to $10.60 retail. Commercial (online) rates are typically 15–20% lower than retail.