I agree - check your airline's luggage limits and excess baggage fees, as most let you take up to 20-32kgs of luggage (plus about 7kg carry-on luggage). That's a lot of luggage, and excess fees can be cheaper than Postage on most airlines if you get close to the weight limit on each extra item.
Maximise your weight limits as much as possible, so that you don't waste the "free" amount you are already paying for with your ticket.