RE: Off-topic: Am I wrong here?
Many sellers will not even send overseas for reasons like this. I think he fulfilled his responsibility when he mailed it, and he's doing the right thing by agreeing to send it to you again.
The mail service in your country admitted a mistake and should not hide behind some international agreement that you and the seller cannot possibly know about. I think it's expecting too much of the seller to ask him to navigate that to get his money back. I think you should demand that your postal service make this right.
I also think it's really good of you to ask here instead of just going into a bitter dispute with the seller. I hope all goes well in the end.
Jim