From what I can gather from the demo, it's quite a simple eCommerce system. I've worked with many of the most common eCommerce packages at several businesses, and they are exceedingly bloated systems that have surplus amounts of features, but the programmers were unable to handle the rapid growth and so they became difficult to work with.
I've not actually taken the time to look for more simple systems for the very fact that clients often want everything that's out there. This is, as we well know, a bad move. But clients will be clients!
What I will say is that if your eCommerce solution does not currently support coupons, then this should definitely be high up on the list of required features, in my opinion. A lot of the clients I've had to use eCommerce systems for, have explicitly specified that coupons is quite crucial -- and indeed it is, because some January time, many businesses want to sell of their unsold stock from the Christmas period, at discount prices!
I will have to download the system tomorrow to have a proper look, but let me ask the question anyway -- is the system coded using any design patterns of sorts, and if not, would you say it's difficult to build add-ons to the system, if I didn't know the system inside-out, as I don't. I think this is another crucial part to any eCommerce system, because the many big and popular ones I've worked with have been seriously lacking in that department, and much like the add-ons for vBulletin -- they end up being merely hacks.
Incidentally, welcome to the community
!