Can you just have the cost automatically reduce in the shopping basket, like the "big stores" do? Like, for example, Best buy & Amazon have some prices that say "Add to Cart to see price", and then if you don't like it, you just remove it. That's how they get around the minimum price deals (or does your online cart even have the capability if you can)? Just a thought - might be easier on you and your email, if possible.
It does sound easier, but it may require more coding..... It's wicked cool of you to do this for us, Eric!
I'm sure that most of the good e-commerce applications already have this capability and it's just a matter of getting it setup. I've looked at a couple of these applications for my brother's business and they can be very complex. I had a difficult time just setting up basic demos for him to review, and I've made my living for the past 20 years programming and configuring computer software.