There used to be a post in the forum about developing CS-Cart such that it can use the shipping cost for motor freight. I sell tables & chairs. The tables are too large to ship by UPS, Fed Ex, etc. A chair order can become too heavy to be shipped by UPS, Fed Ex, etc. too. There are large differences in shipping rates since there are floor prices for shipping such that it may be the same or nearly the same cost to ship (2) tables vs. (1) table. So, a flat rate doesn't work well. Of course, there are also differences in the shipping cost depending on the distance to the customer and the volume vs. weight for each product.
CS-Cart has no interest in developing this any time soon. I would like to split this cost with a few other companies with similar needs. If you are interested please let me know ...
I have this on my site. If you want to know the developer send me a PM. Very reasonable charge. He does not do it as an addon but instales the needed files.
The only problem I had was finding a carrier that would let me use their API. If you do not use them often they will cut off your access.
We've done Estees/Yellow Roadway and custom Fed Ex freight. Reach out if you still have the need. We've created the ability to togle freight over certain LBs .. or in the case of orders w/ heavy and small items - it runs dual shipping quotes where one goes one way and the other goes frieght.
E.g. A order with a t-shirt & a 2,100 lb fountain.