IWP and WebDirect are two slightly different animals, and IWP has been put out to pasture.
IWP was never really meant to be a generally accessible site, there are limits to how many users can be supported, among other issues. That means that every user gets allocated RAM on the server for a session that is maintained for the duration of their session, with a 15 minute timeout, unless they click on the logout button.
In webdirect the same is true but only moreso, and this is also dependent on how many users you have licensed for access.
I would not recommend using this for a web store that just anyone can get to, not even if your expected traffic is very low.
Fully concur with Mike.
There are a couple of things to consider here. Purely from a a functionaltiy point of view you may be better served by one of the existing shopping cart solutions. From a security point of view it may be better to separate the web store from the back-end database.
So whether it is IWP or WebD you may want to take that into account and use a more traditional shopping cart approach..