While we can support shipping rules that depend on the total price of all shippable items in the cart (i.e., not counting the price of digital goods), there's no way to make that only consider specific tangible goods while disregarding others. If you could explain in detail how you want shipping for all your tangible goods to work, we can advise you on what would work and how to set that up. Also specify if you want to ship internationally and what the difference in shipping charges should be for that.
Probably the easiest approach would be to simply configure accurate ounce weights for your products and empty package(s), maybe add a Handling charge for the first item, and have our USPS rate-lookup method(s) obtain the correct rate(s) for the order's total weight and destination. This help page explains our shipping settings in detail: