USPS Airmail is very similar to USPS ePacket. The main difference is the level of tracking for each service. USPS Airmail is a  limited-tracking service. The tracking number has a prefix of UM, and tracking is only available until the package leaves the United States. USPS ePacket features full tracking in the 30 countries where it is available. The prefix is LX, and these packages can be tracked from the moment they leave MyMallBox until they are delivered to your door.