Shipping multiple products in same box using carrier or app to calculate rates

[Nimble]

I feel like I should be able to do this setting up custom shipping rate, but I cannot figure it out.

If someone buys product A alone, I want to charge for shipping, BUT if they buy it with Product B, I don't need to charge them, because product A will fit in the same box as product B.  

I know I can do this if I have a flat fee or set rate, but I use shippit to calculate the carrier rates at check out.

Any advice would be very much appreciated.

Thank you.