The medium flat-rate boxes for Priority service are 13 5/8 inches by 11 7/8 inches by 3 3/8 inches and 11 inches by 8 1/2 inches by 5 1/2 inches. Shipping charges are $12.65. There is one small-size box offered for Priority service. It measures 8 5/8 inches by 5 3/8 inches by 1 5/8 inches and costs $5.95 to ship.