Mixed category shipping calculation

[lot49]

My shipping cart with mixed category products won't calculate.
For my shipping cost calculations I cloned the zones module and have separate product category id's for each. In the shopping cart, shipping will currently calculate for anything in the same category group, but will not calculate for mixed products. I get "Sorry we are not shipping to your region at this time."
I came across this information regarding adding a function to calculate a mixed cart but have no idea where this would go. I guessing you copy from the the shipping class and add it somewhere else. Is this necessary in order for the shopping cart to calculate for products of different categories? Where does this code go?
I would appreciate any help as I've been struggling with this for 3 weeks and have not been able to find an answer that has worked.

thx