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 items that can be fit into an envelope as I mentioned earlier called First Class Mail for LETTERS. Now pay attention, this kind of mail service is a tricky one because it provides a tracking number usually starting with digits (0004) that can not be tracked at all because it won't scan at different post stations just such as the Retail Ground mail service for letters but the main difference between them is the First Class mail for LETTERS travels quicker and have a stamp on it with registered tracking number that is not be able to be tracked which is dropped in the mailbox not delivered hand to hand. So you as a buyer should be aware of this when you order small items to not expect that you will receive a package or an able tracking shipment unless you contact the seller for more information because the seller will write that it provides a First Class mail for shipping without showing the full method. That is all I got and hope people will participate because I'm sure that someone out there has better full idea about what I just posted above.
P.S. sorry for any spelling or typo mistakes because English isn't my first language and I don't actually review what I wrote.
