Determine how users will enter and exit your catalog. There are quite complex and intelligent conditional schemes possible, especially if you use the Cookies capability, but simplicity should be the aim. No one can buy from you if they cannot navigate their way around!
The most important thing to remember is that if you are supporting browsers which might not accept cookies, you must never send the user to a page that is not served by MiniVend. If you do, they will lose their session (and items in their shopping cart). MiniVend will help in this regard, but you must use its area and page tags.
If you are using frames, you must be careful to source all frame panes containing MiniVend links from an initial page served by MiniVend. If you do not, then you may find the user has multiple session IDs depending on which frame the link came from.