They’re possibly taking advantage of the international postage remuneration system.

You ship something from, say, Pakistan. You pay them the postage, they put a label on your parcel, send it to the U.S.

The USPS receive it, label it, deliver it, and note that the original collecting agency was overseas.

Periodically, usually based on the inbound volume, they settle the net value with the other international postal union member.

So - you ship from somewhere porous, with poor systems, it gets delivered, originating agency says “not ours, not paying”, and the USPS is left holding the bill.