I buy a lot of household stuff from Amazon. I also buy a lot of household stuff from lots of other places, mostly bricks-and-mortar.

I don't keep a spreadsheet. I find that when I buy a thing enough times (a particular size of a particular shampoo, say), I get a vague feel for what it "should" cost, and I definitely have a range of values around that expected cost that may be acceptable to me. I don't need a spreadsheet for this. (A spreadsheet wouldn't hurt at all, but it just isn't necessary for me; I don't want to spend the time to get that level of accuracy since the return on my time isn't there for me.)

But sometimes, an item is relatively unfamiliar or the price "feels" somewhat high or low -- whether at Amazon or locally.

When this happens, I comparison shop. I've gotten pretty quick at it.

Charts of historic Amazon pricing for things -- often pretty good but sometimes with large granularity -- are available at camelcamelcamel.com, which scrapes Amazon pricing and has done so for quite a long time.

So I look there to see if Amazon's price is good, relative to Amazon's previous pricing.

And then, if I still haven't found clear direction, I fire up Instacart's website. This lets me search many local stores all at once. (Their prices are often [but not always] inflated, but that's easy to get a feel for as well.)

After that, I have enough vaguely-accurate data to say to myself "Self, this shampoo is a terrible deal at Amazon and I'll just pick some up at the store," or "Self, this shampoo is a fantastic deal at Amazon, and I should order one or two extra bottles while it is cheap."

I wish Amazon's prices were more consistent (why is it $13 this week for a 24-pack of cans of V8 juice delivered to my door, and $60 the next week?), but meh. I can deal with it, as long as I have a real computer or time to fiddle with my pocket supercomputer. It's not so different from conventional "shopping."

I just can't deal with it using an Echo speaker and the limited interactions possible with Amazon's broken Alexa "AI". That's a non-starter, as things are today.

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In terms of quantities, I don't think I've ever been surprised by Amazon. I do not think I've never experienced a thing on Amazon like my parents once did, where they bought mustard at a wholesale club ("Wow, 208 ounces [or whatever] of mustard! What a great price!") and wound up with several hundred individual packets of Heinz mustard loose-packed in a cardboard box.

They were pretty surprised by this. Discussions were had. "What will we ever do with these mustard packets? Should we take it back?
What if they refuse to accept it? Is it even worth the money to drive over there again just for this? We still need mustard right now, too..."

They elected to keep it, and that wasn't the end of the world at all: We had plenty of mustard for a very long time, and it was indeed priced right; it just wasn't packaged in a particularly good way for what we were using it for.

(But then, maybe that experience when I was a kid made a lasting impression on me. And perhaps this was reinforced every time I went to retrieve some mustard from that huge box, wherein: I looked at that box. And when I looked at that box, I could always see that it was neatly labeled from the factory and that the contents wouldn't be surprising at all to anyone who was actually paying enough attention.

It is entirely possible that I might automatically scrutinize the offerings at Amazon, and also at the local grocery store, more than others do.)