I realize this post is 4 years old, but maybe there is an update since I have a near-identical issue:
I have about a dozen products set up for flat-rate shipping based on number of items ordered in the following groups: 1 item; 2 items; 3-5; 6 and up). Each group with their own specific shipping cost.
I want to add a new single item that ships for free and won't bump up the item total. Example: Customer orders 1 original item and 1 new (free shipping) item, the customer still only gets charged at the 1 item flat rate.
If I add a coupon code and have them enter it for the new item, then ALL the items in the cart will be shipped free, and I need to avoid that.
I have a feeling your system won't be able to do this, but figured I'd ask for any ideas you might have. Is there anyway to not allow the new item to be put in an order with old items, thus forcing the customer to place an order with ONLY the new item with free shipping? Would probably be very annoying for the customer and hurts sales, but perhaps is an idea to get started with...
**EDIT: All my current products have the same weight per item; however, the new item is a decal and weighs nearly nothing. I will play around with the weight shipping options and see if that can be an option. I will just have to update every item's information and weight setting. Does that sound like it will work, and will I need to recopy the code on to my site, if so?