ROFL - OK Jas, just for you

In your auction, did you say insurance was mandatory? If so, just reject the payment and send her an email saying why. If not, send her a polite email informing her of the cost of insurance with a link to the USPS website price list and letting her know that unless she pays the full $20, you will send it uninsured. Its her problem, not yours. And if she then complains, make sure you have copies of these emails to provide to ebay just incase.
Hope it all works out for you!
FTY