I am using Zen Cart version 1.3.8, and have made some minor changes here and there, but nothing too major. I have 2 cloned versions for zones shipping, one is called EMS and the other UPS, and both are set to calculate by weight. I recently learned that UPS has this janky system of calculating price that not only depends on weight, but by volume as well.

So, I need to input Volume Weight on all of my products, which is not a problem. I just replaced the name for Product Model (which I do not use) to Volume Weight to simplify things. However, I need to have the UPS zones shipping module I've created to use the Model (Volume Weight in my case) instead of the regular Shipping Weight for calculating the Total Shipping Cost.

I am not new to php, and could probably fiddle around with some code to fix this if only I knew where to find the code to change.

Any suggestions or help that would point me in the right direction will be much appreciated