Great product you have! Really like the flexibility and how easy it is to setup.
A couple of suggestions for the software -
A) recurring payments. The user can select a purchase option, and then select whether they want this ad to be reoccuring (a tick box?). This then means the user does not have to remember to renew whenever the ad expires.
B) a setting per zone for the zone banner target (eg target="_blank") so that any user banners created will open in a new window.
That's all for the moment - I'm sure I will have more