CanadaPost envelop pre-paid, pre-addressed, and ready to clear US customs.
You just need to drop the prepaid enveloppe in a Canada Post box.
You can use this envelop for up to 4 tests. If you have more than 4 tests to send back, you must buy this item for for each lot of 4 tests you have. (ex: 10 tests would require 3x this item)