Bringing additional boxes when flying

What I did was buy Samsonite duffel bags at amazon - they have great reviews and we've since used them 4-5 times and not one has broken or opened up yet during travel. We do cinch the zippers good with extra fasteners...
Anyway so you can probably fit 1 or 2 boxes in the middle of each one then put clothes or blankets or other packing material around the boxes. They hold a huge amount of stuff! Get an extra one and carry it with you and if one is overweight or oversize, take some stuff out and put it in the empty one which will fit in a purse or bag when it's new/empty.

Then pay the extra baggage for each bag. Choose your airline carefully. We found that Delta was one of the best for this. Cost of extra bags and size of bags vary from airline to airline. A cheapo airline often has very stringent limits. I found Delta to be the most accommodating and wiling to help us out when one was a little heavy. The cheapo airlines are the worst, wanting to charge for looking at them the wrong way or because the phase of the moon is wrong. LOL.

We have brought stuff from the US and from MX several times using these duffel bags. Twice we had 3-4 of them and though we were afraid of being hassled at customs upon entering we never have been. They dont' even look inside (though they certainly CAN and MIGHT.)

This is much cheaper than a shipping container and much less hassle as well. if you bring any new stuff take it out of it's packaging and if you need info from the packaging carry it in a purse or carry on, separately.

The nice thing about these duffel bags is they are a size that is allowed unless you really really over-stuff them. The weight is the one thing that might go over so just be careful not to pack too many HEAVY things.

One more tip:
if you have access to a curbside Concierge or whatever they are called at the International flight area for your airline (such as Delta does in some airports) use one and give him $5-10 tip and he can probably get your duffel bags through even if they're a little overweight. They didn't even weigh mine! Best $10 I ever spent. Easier to deal with too.

We divided stuff we need now vs stuff we want in the future, stored the stuff we want in the future at a friend's house and will bring it down bit by bit as we travel to the US and back. On our first trip I think we had 2 regular suitcases and 4 duffel bags. They never checked inside any of them other than with the xray machine.