How can I integrate FedEx rates with different box sizes per product?

Hello,

We have negotiated rates with FedEx and will be integrating our account. Currently Shopify uses one default box size when calculating rates.

Does anybody have experience with integrating negotiated FedEx rates and know if you can set box sizes per product to accurately get calculated rates? We’d rather not pay for an additional app if possible. Just want to make sure before we do.

Did you ever find a solution for this problem?