I lived overseas for a number of years, and it was common to have postage cost more than the item. (hence the $1600/3 months figure .... I was buying in bulk in order to make the shipping handling costs go down)
Over here, we've started buying some groceries from online vendors because the shipping is free (or nearly so) and no local taxes apply. Amazon.com now sells just about EVERYTHING! It seems to be selling MOST items now with FREE "super-saver" shipping, and since we live in a remote area of Texas, it's a LOT cheaper to buy online than drive to the nearest city and waste the gasoline and time.
By the way.. I have no way of checking to see if Vitacost is the cheapest online. There may be better deals, but I just don't look for them until I'm unhappy with my current vendor. The Google "Froogle" search engine used to be pretty good for searching vitamin deals, but they changed the layout for some reason. I find good deals on Ebay, but you never know what you're going to get when buying from the Ebay boys. (probably nothing you'd want to put in your mouth)
After more careful consideration, I've realized the flaw in suggesting you move to the US. I forgot the cost of (slapping forehead) AMMUNITION! The costs of dinner, a movie and a good shootout with a carjacker are skyrocketing.
One pallet of double ought buckshot really puts a dent in your vitamin shipping offset, you know.