I definitely second the info about post office clerks trying to talk you out of more expensive Priority International shipping! (But, oddly, really pushing domestic Priority shipping.) I think a further complication is that nearly all international buyers DO choose parcel post shipping. I fully understand why you want Priority International instead, SoSilver, but if a seller is used to shipping by parcel post, it's easy to make a mistake.
In my opinion the seller deserves a positive. He made a mistake, but has gone above and beyond trying to rectify it. I think Stormy's idea is a really great one!