I've done some purchasing of Lego on the net and have a decent collection of my own. In my experience, Lego that's put together is far more dense than the box it comes in, so my advice would be to ditch the packaging and put the pieces together - not necessarily as per the instructions but in as tight a format as you can. It won't affect the weight of course, but it wont take up nearly as much space in your luggage.
If you weigh it while you're there, you can always weigh up whether it's cheaper to take it in your luggage (if you're carrying a chargeable excess that is) or to post it home.
Sounds like a great trip! Hope you enjoy it mate.