I have to agree with redneck and Jester on this- and I know, I'm just repeating what they've already said more efficiently. But to go to an outfitter, use the service they offer by having them spend their time outfitting you, answering questions, etc... then say so long, and save five bucks by buying it online is not cool.
Besides, packs, like Birkenstocks, can occasionally change size very slightly from one to the next. And you might as well learn how to adjust it to your body from the outfitter. And you'll have to pay shipping. And if you buy it from the outfitter, you'll have it right away, which is SO much more exciting than seeing it, and waiting to buy it.