you're going to have to jump through some hoops...
I recently had the EXACT same thing happen to me. While it's not a lot of money, I found it kind of demeaning that they would make me buy the product for $80 and then start offering up incentives as my "trial" was readying to expire. Here's what I went through but be aware, your mileage may vary.
I called customer support (twice). The first time, the guy really didn't seem to care too much about my plight and I didn't get any traction with him. The second time I called in, I asked to speak to a supervisor. The first rep was highly resistant to sending me to a supervisor, but I wouldn't let him off the hook. Finally, I got a supervisor on the phone who explained that they were unable to refund the difference to my account. He ended up refunding the entire amount to my credit card and asked me to re-purchase the software using the discount link.
For what it's worth, I have a Frys Electronics here locally and the week this happened, they started advertising Parallels for $40. Long story short, I ended up getting the refund from Parallels for my original purchase and then I bought a boxed copy from Frys and just registered it online. I never had to uninstall anything and all my VMs worked fine (until I upgraded to Lion, but that's another story for another day).
Good luck!