Seller is now asking for increase in postage after payment was sent!

[elmomax]

I purchased a mug for $12 and shipping was shown as $8.40 for priority shipping USPS. 

After paying for the item, the seller contacted me and told me the shipping comes to $12.50 plus $2.50 for insurance ($6.70 more than what the ebay ad indicated and what I paid via paypal.

Seller is asking me if I want to pay the additional amount or cancel the order.  I tried to reach Ebay customer service with no luck.  I do not think a seller can change the price of shipping/insurance after an item has been paid for.

Please advise.  I need your help. I am **bleep**.

Thanks in advance for your help.