Refer to the Ts&Cs; I assume you'll have the details of the retailer (and their own Ts&Cs) as part of the order process:
"If, on receipt of the item, you find it is unsuitable, please follow the instructions according to retailer concerned, received in the parcel. It is helpful to inform Delivery Code that you have returned an item unless it is a straightforward exchange. If an item has been returned, Delivery Code will issue Delivery Code vouchers reflecting the amount refunded by the retailer. "

(sorry i realise this isn't relevant to your question but they have gone from being really good to lately, total scam artists.)