So, assuming they've sent you what you asked for...
Hmmm, the DSR regs are there to allow you the same opportunity to check goods as if you were in a shop. You wouldn't fit a tyre in a shop to check sizing would you? You'd be able to tell it was 'too big' by looking at it.
If you've not exercised the pre fitting caution of even a cursory inspection then why do you think the shop should 'carry the can' for you ordering something you've now changed your mind over?
If you now don't want it, repackage it as new and send it back under DSR (i.e. extract a digit and get the right cable tie - tssk) or throw yourself on their mercy and see what they say...
Asking them to pay your return costs is really taking the pi$$ when you've clearly cocked up.