Priority Mail Flat Rate® Medium Box - 1 | USPS.com
TIL: when shipping brass, bullets or lead, 6 USPS small flat rate shipping boxes fit perfectly in one medium flat rate box. Great reinforcement. : r/reloading
If it Fits it Ships Box Sizes and Prices Detailed - Geniuz Media
U.S. Postal Service Announces New Prices for 2022 – Small Flat Rate Box Rises $1.00 Dollar to $9.45