I am guessing one can have a prepaid FedEx, a UPS and a DHL envelope while traveling. This will guarantee that at least one service would be available, but this option might be too cumbersome.

From the chckpoint site, it looks like they want it dropped in their boxes specifically:

1. At a CheckPoint Mailers site, pick up one of our 6"x9" plastic pouches.
2. Complete the payment and shipping form attached.
3. Enclose your item and completed form in the pouch and drop it in our box.