I believe when I've looked at it, the prices are no worse (and should be better) than those offered by GardenGrocer. And should be close to WeGoShop, but without the service charges/fees that they would charge you. The biggest problem is that it's pretty much all non perishables, so if you need milk, yogurt, etc then its a tougher sell.
They have 2 factors in play. They account for both the size and the weight. Those are both pretty heavy items (as well as large) which is what fills it up so fast. Basically it means you don't have to ship snacks and stuff down separately, or pack them in an extra suitcase.