You must make sure that your items fit the box you have chosen and do not exceed the weight limits. There are several kinds of boxes and the maximum shipping weight for the largest one is 20 pounds. The shipping cost for Priority Mail International Flat Rate Boxes is predetermined for this method and does not depend on weight or distance. If you choose to use Flat Rate Boxes you don’t need to worry about weight of the package.

Priority Mail International Flat Rate Boxes Priority Mail International retail pricing rates start at $26.45 when shipping an item weighing one pound or less to Canada and increase depending on the weight and distance. Please check Countries and Limits for sizes and weights specific for each destination. Oversized packages may be subject to a surcharge. Maximum combined length and girth (girth is the distance around the thickest part) sent by this class to most countries is 108' and maximum weight of the package is 70lbs. When you ship your items via Priority Mail International retail pricing you use your own box. Priority Mail International Package Shipping

Usps flat rate medium box dimensions