Add free shipping to specific items

My client sells signature opportunities with celebrities. People buy a signature, then send in their item to be signed. Once signed, its returned to them. They pay for the return shipping during the checkout process. Sometimes though, there is overlap. Now you have signature 1, and signature 2. They will announce a new celebrity signature (signature 2) that is related to a previous one, and the item is already in the client's possession (for signature 1). The customer is now charged shipping twice for the same item to be returned. We need a checkbox DURING THE ADD TO CART process that a custom can select if they are adding a second signature to an item thats already in our possession. This will remove the shipping cost but ONLY FOR THIS ITEM. When selected, the customer must also then see a textbox that allows them to enter their previous order number (to tie them together).