Carbon intensity by shipping method
The World Shipping Council provides this chart showing approximate carbon emissions by shipping method:

Shipping emissions calculators
If that's too approximate, there are a number of different calculators you can try out:
The last tab calculates annual emissions based on quantity, weight, and distance of packages shipped. It's annualized, which is annoying, but it's the only one of the three that includes air freight (and zeppelin!).
OOCL is a Hong Kong-based logistics and shipping company. This calculator breaks down results by shipping method, but only seems to work for major ports.

CN is a Canadian railway operator. This calculator seems like the most useful of all. You enter the major ports for shipping distances, then add additional distances that the cargo will travel by train or truck.
