So, where is the case against USPS? Clearly they are the ones at fault. Also not an isolated incident.

Does this really happen? You take the package to the post office, they weigh the package, charge you money, give you a receipt, and then later demand money from the recipient in order to finally deliver the package?

Nah, the package isn't weighed twice. More likely than not the seller weighed it themselves and used a calculator (either on eBay's website or with an app like Stamps.com) to figure the amount, which was off by a bit.