I was selling some things on Facebook, advertised as free local collection.
Someone messaged me saying they wanted the item and please could I post. They paid postage costs (£4) and I posted the item this morning and sent them proof of postage.
It was 50p cheaper than they paid and they asked if I’ll be sending them back the 50p. 
I explained that I’d needed to buy the paper to wrap it specially as I had only listed it locally and had not planned to post. I also explained that I’d had to take a 3mile round trip to the post office to get it sent to them.
AIBU to not refund the 50p? What is an acceptable amount above the actual cost of postage? 50p? £1? (I had estimated the price online based on the size and weight of the object.)
YABU: you should refund the 50p
YANBU: you shouldn’t refund it!