This is probably a comparatively small issue, but I'd really appreciate some clarification/advice.
I recently bought a couple of vintage posters (and expensive they were too 
) for the playroom from an independent shop via facebook. They quoted £15 for postage. Just got a message from them saying postage was less than they were expecting and they will give me a £5 credit for their shop because of this. Thing is, tbh I'm never going to buy anything from them again so the £5 is effectively useless. Am I at liberty to tell them this and ask for a refund of the £5 overcharged or am I just lucky they are honest and offering me a credit note?