Right. This was an intentional attempt to cheat the system that the seller fails at, repeatedly. They attempted to screw the USPS and the customer got the bad end of their incompetence.
>This was an intentional attempt to cheat the system
You don't know that. Say the seller has a faulty scale and does large numbers of transactions at once. 100 packages go out, and 100 postage due complaints come in.
A non-scumbag would resolve the issue for the customers and fix the systemic issue. That this continues to be a problem is a sign of a bad seller. The first time this happens, you need to re-calibrate your scale.
In their defense, at least they offered to refund the extra shipping cost, but relying on not being caught to save a small amount of money is not worth it in the long run.