From my Experience, When customers ship different shipments from U.S, the USPS shipping company will show up as an option to deliver your shopping stuff. Sometimes, you buy a small item that can be fit into an envelope and the seller provides the First Class mail as a shipping method to this small item. We all know that the Fisrt Class mail service from USPS comes with a tracking number that is able to be tracked but there's also a service mail from the First Class Mail especially for small&nbsp...

Tracking # starting with service code (0004) from USPS shipping company | Ask a Mentor

Tracking # starting with service code (0004) from USPS shipping company | Ask a Mentor | eBay Community