I had something similar happen once. The seller sent my win in two seperate envelopes as she figured it would be cheaper. It arrived with $8.00+ postage due. I went into the post office and had the clerk show me the packages, weigh them, compare the weight paid for and the actual weight. I thanked him, and let him know I would be back. I left without the packages (money was due on both) and immediately contacted my seller to let her know there was an issue. She was very polite, though I'm sure not pleased. The problem with my packages was there was a discrepancy between the USPS scale and hers. She agreed to reimburse me the extra postage I would have to pay. I bailed my packages out the next day, and she immediately repaid me. I of course offered to scan my receipt and send it to her to verify that postage was indeed due, but she declined. Everything turned out well in the end. Ultimately it is the seller's responsibility that appropriate postage is paid. I would contact ebay and follow their recommendation like I did. Good luck!