I have no special insight into the financial workings of Amazon, but I have come to believe that a meaningful portion of its profit comes from shipping charges. If you select free shipping, they wait up to a week to pick your items before shipping. If you select standard shipping, which uses the same conveyance as free shipping, your items are picked, packed, and shipped within a couple of days. Amazon's shipping is the online equivalent to putting impulse items at eye level at the checkout.
Note:
This is not a rant or complaint, just an observation.