I know this might seem petty but it's more the principle of it rather than the actual amount.
I bought a load of items from the sale seller on eBay, all with £2.50 postage. I asked for combined postage and she charged me £5 to post the lot - great!
The package arrived and the whole thing had only cost £2.50 to post.
I asked her for the other £2.50 back that I paid her for postage that she didn't spend and she said no she had already given me a reduction!
She is missing the point that if I pay £5 for postage I expect it to be posted at that value and arrive quickly, not take ages to arrive 2nd class for half the price I paid.
As stated above, it's more the principle than the actual money.