Partial refund for shipping?

[3islanders]

Related to my other question about selling expensive items to the USA. If I have to use UPS to get insurance coverage, the cost of that will vary on the final selling price. I wonder about making a flat rate based on average sold prices and the farthest state from me, and offering to refund excess.
How would I do that? If eBay holds funds until the buyer gets the item, then I assume the refund would have to be after that...?

[pjcdn2005]

You can refund through eBay before you receive the money.  Refunds can be done from pending payments.  That's how it works if you are buying shipping through eBay as well...shipping is paid from pending funds.  If the funds are deposited before the amount is deducted from pending payments, then they will bill your secondary selling fee source.