Add the ability to give one free option Oscommerce

we want to add the ability to give one free as an option in the oscommerce coupons code module that we have installed. Bascially, when selected for a coupon code, the customer would receive one item free, he would still be charged shipping, but the the lowest priced item in his cart would be free. If it was only one item, he would only pay for shipping. We would want to make sure that it doe snot mess up the shipping module as we plan to offer buy x number get free shipping as well, such that if they buy one they would only pay shipping, if they bought 2 items, the lowest price item would be free and show as the coupon amount on checkou and on the right panel cart, and it we make it buy three get free shipping, if he bought three he woudl get the first item free and free shipping. The free shipping module is already installed and working, this is just a change to the coupon module which would entail an admin change, checkout change, and cart change as well as adding a field to eh table for this option.