Have you looked into MS RMS?
There were a couple local shops I noticed using it for a while. It may not be 100% what you're looking for, but there are lots and lots of off the shelf mods for it to do this or that.
It does customers, inventory, shipping, ordering, invoicing, etc. It's all pretty easy to modify also.
I have a stack of CDs of 2.0, you can install it and it'll run for 30 days without a key. I can also give you a contact of my local guy I talk to, and the support team that's in Portland. They are a great vendor and one of the few companies I'd recommend.
Just drop me a PM or an e-mail and I can mail a disc up, or you can pop up to the Pub for beer night and pick one up.
At least worth playing with for a couple of days since it won't cost you anything but some time.