Rules: I only buy it if it’s Prime eligible, otherwise it’s skipped. If I bought something because it’s on sale, I take the lowest price amazon ever offered for that item (with prime). Otherwise, I take the price of the item on the same day.
Results: https://i.imgur.com/kDeM2pM.jpg I saved about $330 by not having a prime membership and buying stuff only from prime eligible stores. However I spent at most 27 more days waiting for items (in reality a lot of these shipping times overlapped).
So the takeaway is that amazon made things easier and faster but not cheaper. I know a lot of people still think amazon is the cheapest place to get things but that hasn’t been true for a while.