As of 2014, the cost to ship a USPS medium flat-rate box is $12.65, which includes delivery to all 50 states. There is a maximum of 70 pounds per box.
The priority medium box comes in two sizes, one long and flat, and the other short and tall. USPS medium flat-rate boxes ship as priority mail. This service includes USPS tracking, insurance and delivery in one to three business days. Seventy pounds is the domestic weight restriction. The boxes are free and available at all USPS retail branches. It is also possible to schedule a pickup of flat-rate boxes from home or business on the USPS website.