I'm sure that would be the case as at the end of the day, you've only bought one item/set.
I've just received the Eternal by diamoneek bracelet tsv (post round here has taken 3 weeks to arrive for some stuff I've ordered from elsewhere so this was quite quick). I ordered one in each colour and there was a £3 discount for the second and subsequent ones. The invoice for one shows £15.75 and the other two show £12.75. I assume that if I kept the two 'cheaper' ones and returned the dearer one, I would only get £12.75 back and not £15.75 and that's only fair.