Hooray! I think...
Here's my scenario:
I have Music CDs for sale and Downloadable MP3s. I use the USPS Shipping Module for the shippables and want to use FREE or N/A shipping for products that are downloaded. OK...
1. The product weights for my MP3s (FREE SHIP ITEM) are ALL set to "0".
2. I created a new Shipper named "FREE" and a new Shipping Rate also named "FREE", with a rate of "0".
3.I assigned the new Shipper "FREE" to the Shipping Rate "FREE".
4. Finally, go to the "Admin/Configuration" Shipping TAB and enable the "Standard Module (RECOMMENDED" along with USPS and any other that apply to you.
I assumed it had to be one shipping module or the other. But it can be as many as you need.TESTED:
1. I placed an order for 3 MP3s and shipping came up at FREE - Yay!
2. I placed an order for 3 MP3s AND a mailable CD and the shipping came up ONLY for the mailable CD with no extra added for the MP3s.
I hope this helps, so much so, I've included screen shots. I have STRUGGLED with this for WEEKS!!!!!