You really have to check with each individual cruise line to see what their practice is. Also these practices change so get it in writing when you make a deposit.
Also call the airlines and find out how much they charge for excess baggage, it may be less expensive just to take the extra luggage with you as extra baggage. Various entities use air couriers paying the airfare of someone so they can get the passenger extra baggage rate for their shipping rather than just shippng their packages.
If shipping isnt practical or possible dont worry you can pack light. On deck on sea days you'll mostly wear a bathing suit all day and you really can rewear your clothes on the cruise. You dont need 108 different outfits. Another thing to check is whether the cruise line either offers laundry machines or a landry service to passengers. It may be more practical and less expensive to have your laundry done once in a while rather than have enough clothes for 108 days.