seller undercharged their shipping price they posted now they want more money after auction ended

[wonderwomancollector]

i bought a stereo receiver for $25.00.  Shipping price posted in ad was $17.44. That comes to a grand total of $42.44, which i paid with paypal. The guy goes to post office to ship item...they charge him $39.00 shipping. He then sends me a message asking for an extra $10.00 because he misjudged the shipping on that particular item.  What should I do? I paid the amount listed in the original add. I dont want to pay an extra ten bucks. Just looking for advice on the correct way to handle this. Thanx in advance.