Flat Rate Envelopes are ideal for documents or small, dense items weighing 4 lbs or less, as they offer a single price based on the destination country. For items between 4 lbs and 70 lbs, or those that don’t fit into the Flat Rate Envelopes, you’ll need to ship by weight using your own packaging. It’s more affordable to use Flat Rate boxes if your item is under 20 lbs.