Shipping on Customer's Acct: How to require customer acct number?

Helpful! I got this all setup except for one part, please forgive my ignorance

All my products are assigned to the “General shipping rates”

My free shipping option is setup under “Custom shipping rates”

But for the free shipping option to populate I have to move products from being assigned to the “General shipping rates” to the “Custom shipping rates” and the problem is then every time these products are in the checkout they only populate the “Custom shipping rates”

How do you manage what shipping rate your products are assigned to?