I wonder do they ban people’s accounts for doing such things? That’s basically theft

I think the term is "mail fraud", and it's very illegal

Is it mail fraud if it does not go via USPS?

The wiki page on mail fraud quotes the definition and it would seem to include the USPS as well as any private/commercial carrier crossing state borders.

That’s interesting. In WA for example it probably won’t cross state borders if it’s a common item