Last time I ordered from them, they sent everything in just one box.
I forgot to mention that I was talking about Amazon's Global Priority Shipping, which is $10 - $12 per book, no matter the size, if you make a big preorder. If the books have already been released, it's much more expensive. That's why I think a $3 - $4 shipping charge (after you pay the initial $25 - $30) for Deluxes is pretty awesome.
For example, in IST, the shipping cost to Argentina for the 4 Before Watchmen Deluxe Editions is $28.60. At Amazon, using Global Priority Shipping is $57,95 and Expedited Shipping is $42. If I had the four books on preorder, using Global Priority Shipping, I think it would have cost around $44.
Now, why do I say you have to experiment with IST's system? Here's why: the shipping charge for 1 Before Watchmen Deluxe Edition is $24.77, for 2 Deluxe Editions is $20.66, for 3 books is $25.97 and for the four OHCs is $28.60.