OK, I just though of something that maybe others might not be aware of. I kindof assumed everyone knew, but maybe it might help:
The commodities produced by a city change as the city grows. If you have fast-growing cities like in a Democracy, it can really pay to PRB (Partial Rush Buy) the freight with the commodities a small city produces, and wait until the city grows about 10 citizens in 10 more turns before delivering it. The cost of buying this freight (don't build it.... buy it) is typically very small compared to what you'll get. A great example is Copper. I used to wonder why so many cities want it, but only a few make it. Its usually made in smaller cites... if you wait until the city is size 20 making 50 shields a turn, Cu probably won't be made anymore. So if you like a commodity a small growing city is making, buy it and deliver it when the city is bigger!!