How to make the whole cart for FBM order free shipping if one item purchased has free shipping?

I’m struggling to set up free shipping rules for FBM orders with multiple items. I have FBM ASIN’s with free domestic shipping, with its own shipping template, and some ASIN’s with paid shipping with its own separate shipping template. This is a “main item” (free) and an accessory (paid) situation. I want to have it set up so that if a customer orders multiple items from my FBM store, and at least 1 item in the cart has free shipping, then the entire order is free. The accessory fits in the same box as the main item, its low cost/low margin which is why the shipping is normally paid if purchased alone. Presently, the shipping charge is the sum of shipping of each of the items in the cart. How do I set this up? I had a chat with Amazon Seller support, but they said its impossible to do, but I wasn’t exactly convinced of their expertise. Is this possible? Thx