I sell clothes and things on ebay. I typically add a blanket postage fee of £3.20 - the cost of a 2nd class package. The only time I don't do this is when I know it will cost less than a £1 because of how thin it is.
Around 98% of my sales are small package size, so at this point I have charged the buyer exactly the postage cost, but even then I will be out of pocket as I'm not adding extra for packaging and final sales fees.
Occasionally I will have got the estimate wrong and undercharged (I never ask the buyer for more money to cover in this instance) or it does actually goes through as a large letter.
In this instance, the buyer actually bought two items with the £3.20 postage charge and I combined the postage costs (and I did this without being asked as a goodwill gesture) and put them into one package (so she was only charged one postage fee). I was charged £1.53 at the post office. The buyer is asking me to refund the difference.
When you buy something from anywhere else that has postage, you don't demand a refund on it's arrival based on the actual cost. Why do it with a private seller?
AIBU to say no to the refund citing all the above?
(By the way, the total cost of the two items was £6 so it's not like I made a killing on them anyway after fees).