The other thing to think about when refunding overpaid postage (if you include it in your listing price) is that eBay have already taken their FVF out of the total amount including the postage.  So if you send a refund you are out of pocket whatever the %age of fees you pay to ebay.  So for each $8 postage you refund your customer you may lose 80c if your fees are 10% of fvf.

Sounds like it would be cheaper for you to keep postage separate to avoid over payments.  Which means eBay will still get that fvf but only on the correct amount of postage paid.