If the retailer sold you underlay and carpet , and you don't need the underlay because it is already part of the carpet, then you may have a case.
If you don't need it because of something they didn't know about then it is not their fault. Having said that, it sounds like you asked them to cancel the order almost right away (did you?) It sounds like they are being especially harsh.
If they haven't yet delivered the carpet, then I would have thought they would be more accommodating.
This is not specific advice, but if you look very closely at most carpets, you should be able to find a fault that would justify it being returned as "faulty". If they have any sense, you would think they would try to avoid upsetting customers unreasonably!!