Dear David,
If nothing comes of your swap idea, I would suggest asking for eBay's help with this. It is after all the Sellers responsibility to get your purchase to you properly and as promptly, if something goes wrong, then it is the Sellers job to put it right. They can't just walk away waving their arms about saying they can't do anything - very particularly when you have already paid them money. If they had the postage rate wrong that's their fault... they should have checked this when it was sent... if, however, the PO where it was sent from did not charge sufficient to cover, then it is this PO's fault and they must put it right, however, the Seller must do some jumping up and down and rectify this. I am sure eBay mediation would resolve this problem for you.
Hope you get it sorted out soon.
Regards, Bernard