You can find cost adjustment details on this page: https://www.ebay.com/ship/lmng
Cost adjustments happen all the time. It's a pain in the ass, but it's easy to appeal. Send USPS an email verifypostagehelp@usps.gov with the following details (or you can call the number on the USPS page linked below):
In order for USPS to investigate your dispute they must have the following information:
- Full Name
- Phone Number with area code
- Email
- 10 digit Revenue Assurance ID or USPS Adjustment ID of each record to be reviewed
- IMpb or tracking number
I generally describe the problem, as well. Shipped 8 oz t-shirt in plain white polymailer with eBay logo, was up charged based on a Priority Flat Rate Envelope packaging which was not used.
Packages sometimes get stuck together or are too close together when going through the sorting machinery so the scans are reading a different package. When you file an appeal, USPS will look at the actual photos taken during sorting and they will see the package and grant the refund.
USPS sends the refund to your original source of payment (credit card, bank, eBay funds, paypal, etc).
https://faq.usps.com/s/article/Automated-Package-Verification-Program
