Hi,
Thank you for the comments. Unfortunately in this instance, we require that everyone can access the shop and purchase most of the products, there are only a small handful of products which should only be available to members. As a result, the DNN permissions system cannot be used, and requiring a login wouldn't help (since both members and non-members have logins).
I think that the solution I need would be some sort of access restriction on the products themselves, which doesn't appear to be supported at present (admittedly its an unusual requirement).