Within the Amsoil model, the cost at your corner auto supply store should be close to the online price. The store would sign up to be an Amsoil dealer and they would get discounts on the product and then they put it on their shelfs at a price. I don't know the current arrangement with all that (such as if they can sell it for whatever they want or if there's a MSRP they need to stick to), but they are not buying the oil you need at $15.39 and then marking it up from there.It's possible.
If I dont purchase Amsoil, though, it won't be because of him. It'll be because they charge $15.49/quart for what I need, as well as I have to apparently order direct. Oddly, this is a practice that typically saves the seller money. I shudder to think what it would cost me at O'reilly's to purchase Amsoil if they ever carried it. $20/quart?