Considering that "shop.com" doesn't comply with UK Consumer Law (and in fact seem to think they can trade in the United Kingdom without UK law applying to them) I'd give that one a big miss.

Always, where possible, trader directly with the retailer. Trading via third parties creates a hell of a mess legally.

EDIT: I've just checked chemistdirect, who score big brownie points. They mention your rights to return under the DSR, which is good going (and legally required, but hey, don't tell shop.com). I'd recommend them.