From what I’ve seen, Amazon is already cheaper than AAFES. The last time I did a comparison of some items I was interested in, Amazon ran 10-40% less, including shipping.
We use the commissary all the time and it does have considerable savings over the grocery stores in town. The exchange though? I am very curious to see where they are pulling this 25% savings number from because that has never been my experience. Their brands maybe, but not overall.