Being charged a different amount than when package arrives

[cdv72]

Will Ebay investigate a seller who overcharges you for a package? Meaning, they mailed me a product in a Priority Mail box with a label that says it was a flat rate envelope. So, they paid the post office $6.96 for a flat rate evelope sent in a priority mail box and charged me $14.95 to ship. Or is it just the post office's loss?

[brownwrapper]

Did it arrive with postage due? If not, then it's a loss for the usps.