That's an interesting one. If anyone's not looked, fivetrees.com is a complete mail order service that holds stock and wraps and sends the goods for you.

If you want a minimum cost UK online payment service and don't mind doing the actual packing and posting yourself, I can recommend NOCHEX. You have to apply for a "seller account" but I didn't have any trouble doing that just to sell folk CDs. All types of plastic accepted but only on UK banks. Much cheaper than Worldpay and PayPal, and you get your money reliably and immediately, unlike some other services.

Anahata