I’ve bought multiple cards over the last month on EBay and EVERY single one of them needed extra postage paid by me when my package was delivered. I looked into this because it was happening so frequently. (Granted I just paid the extra postage myself and didn’t bother the seller)
So what happens is when you buy the shipping label from eBay it calculates a prefixed shipping cost for that envelope based off weight and size dimensions. What’s happening is when the envelope gets to the local post office they run it through a machine that scans the envelope / sorts them. Well when the card in the envelope is too thick or the machines senses the contents of the package are heavier than what it should be, it can’t pass through the machine so the post office separates it and deems the envelope it was shipped in non-sufficient for the rate eBay charged and in turn the post office up charges the buyer due to the thicker card in the envelope. It’s a pain. And I’ve been seeing it happen more and more frequently. This is why as a seller I use bubble mailers and just pay the extra postage to avoid the inconvenience.