Well, I finally got this to work but it sure wasn't easy.
1) Yes, I had to spend an extra $200 to buy DataAssist which doesn't make me real happy but I'm sure I'll find other uses for it some other day so it's not a total loss.
2) In all honesty, the tutorial you recommend is worthless. I'm sure it had value when it was written but it's badly in need of overhaul now. Not only does it reference a sample database (Blue Sky Footwear) that no longer exists but it also covers just one multiple item scenario, that is, when you present your user with a column of optional checkboxes next to your product listing.
I needed that scenario but I also needed to be able to put multiple items in the cart *without* checkboxes too. In other words, I have a "bundle" of products that go together unconditionally. I didn't want to make my users click on five checkboxes every time. I just wanted one Add To Cart button to dump five products into the Cart. No other thought process needed.
Anyway, I got that to work too. If anyone wants to know how it's done, drop me a line and I'l be glad to help. In the meantime, if the Webassist team would consider it, I wouldn't mind re-writing the tutorial for you if you're short on resources.