I was going to advise about the proverbial 'small print' but I see it has helpfully been posted already.
It is frustrating.
I encountered a similar scenario, returning two dupl;ciate books Mrs Hughes and I received as gifts, only to be told that they were now discounted to half the original price, which is what they would offer me as a refund!
I understand there is nothing I can do about it, but I was not impressed - it means that our relative is unable to have us receive gifts at the value she wanted, and if we want books to that value, we ,must may the additional cost.
Happy New Year Waterstone's!!