Special Order for a Set of Items & Supplier - Customer Discount Link

Hi everyone, one of our suppliers has introduced discounts for some of their items if they're bought in certain numbers and configurations. For example, if we buy 3 x item A and 1 x item B, we'll get a discount + item C free of charge + extended warranty. Unfortunately, the nature of these goods prevents us from creating a new inventory item to include it all and the only way I know to accommodate it is by creating a set. We don't stock these items, but we also don't do drop shipping and everything we sell needs to be