The business model seems to be that they hold no stock, get the orders in, then acquire it from the suppliers in one fell swoop. Quite neat as they don't end up with warehouses full of unsold stock. It does mean that they can't handle returns though.
FWIW, I've bought a fair bit of stuff from them. It all turns up eventually, just a few days after the offer finishes.
Shame none of the Icebreaker tops are short-sleeved though. The last batch of 3 that I bought are on their last legs (mind you, they are now 3-4 years old).