Special shipping rates for a coupon code

[Ivan Santos]

Good morning all!
We currently have special shipping rates depending on the order's total amount, and we apply the Standard Courier shipping rate for orders below a minimum amount. We were about to start a Buy one and get one free promotion, offering a discount coupon code to our old customers, and then after some calculating, realized that we were going to lose a lot of money in some shipments(especially to the West coast, we are in the East...), if we were to keep our special shipping rates, because of the fact that the package's weight would increase double. My question is the following: would it be possible to cancel our special shipping rates on this promotion, and only apply the Standard Courier Shipping rates(based on weight and zip codes), and keep our special shipping rates on regular orders? I mean, if a customer uses the discount code he would have to pay the full COurier shipping rate. But if he doesn't use the coupon code, he would benefit of our special shipping rates.
I made a search on this forum, but the only thing I could find is something called Custom SHipping Modules, I have no idea of how it works...
Thank you very much!