I have had two different sellers send me items in a flat envelope that required me to go to the Post Office window an pay additional postage. If there is something that is too thick or rigid and the PO employee feels that it may get damaged in the machine, they cancel the stamps by hand which is an up charge. This will stop the envelope from being delivered.
If it does go into the machine, there is a possibility that the rollers will get caught on what every is inside and damage the envelope. If the address gets damaged, they do not know where it goes.
Sounds like your card may have been hand cancelled and the buyer will have to pay the additional postage after they stand in line at the PO.
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