Greg, to be absolutely fair, I don't think you will find any cart with the flexibility you desire which won't require at least a small amount of programming. You can always do easy straight forward implementations without coding but adding flexibility adds complexity.
My personal feeling is that for some reason too many people come to the forum with the expectation that "no programming is required."
I know it's very frustrating and it will take longer but learning a little programming will help you to accomplish spectacular results. One book I would recommend is: "PHP Solutions" by David Powers.
You won't "find" a session variable you will have to code it. They look like
$_SESSION[something here] The code for the different rates can all be generated by WA. It may not be as straight forward and walking through a wizard, but the "heavy lifting" can be automated.
Again, I would highly recommend you get the book. In a couple of hours time you will know a LOT more and probably progress your project faster than the approach you've been using.
I feel your pain, I think we've all been there before. Hang in there.
ps. I like Ray's session variable approach even better. I tend to do too many things by hand.