Shipping prices generally are interesting. I get that shipping isn't free, but often enough there really would be potential to optimize. E.g. Amazon ships what they can in small cardboard envelopes (stuff like USB drives, small cables, electronic parts, that kind of thing). Efficient and fast. Other sellers want 4.99 € shipping for that. Which, if I' buying stuff for 15-25 €, is a notable percentage of the price. (And yes, if you send me a big shipping box in which I have to dig through the packaging chips to find the product, then you probably paid close to that for shipping, but that really wasn't necessary)
Or the one that also has stores, where you can either pay 5.99 € shipping or do store pickup. And I'm thinking "eh, the price is a bit cheaper than everywhere else, but they want more shipping. BUT I can easily pick it up myself and maybe even get it a bit quicker.". Change the shipping method, and discover that it now charges me no shipping, but increased the product price by 5.99 €. And which point I ordered somewhere else, didn't go to their (big, probably expensive) store and didn't have a chance to be tempted to buy some additional stuff there.
At work I often ship ceramics [in the UK] - none of the couriers will insure it, the reason I'm told is that they all use overhead conveyors and so all parcels have to survive a 2m fall. Perhaps that's part of the reason for "overpacking". Another could be that it's observer bias; occasionally they run out of smaller boxes and so use a large one (necessitating lots of filler) and these occurrences get reported.
The way I buy online, Prime doesn't save me any money. I always go for the free shipping. Without Prime, I wait longer. Prime only saves money if you routinely pay for faster shipping, and do a lot of people really do that?
Walmart.com offers free shipping for orders over $45. Target.com offers free shipping also. And if you are ordering dog/cat food or some niche thing, ordering from Chewy.com or computer parts from Newegg, etc.. will net you better prices.
I'm not sure if its Amazon who's canvasing online forums or the myth just continues to get perpetuated by less than savvy shoppers. But Amazon is hardly cheap unless you really do need the 2-day shipping. Which most of the time, people don't mind waiting 3-5 days to save 10-20%.
Here's an example -- I found a Kingston SSD drive on Newegg that has a Limited Time Offer "See price in cart" gimmick discount. It's $1.10 more expensive on Amazon. I found another Newegg limited time discount Samsung SSD drive that's actually $0.76 cheaper on Amazon. You are not going to be saving 10-20% by waiting 3-5 days.
Juggling logins and shopping carts across a half dozen different websites isn't convenient. Coupon hunting isn't convenient. Amazon Prime is all about convenience and it delivers.
So your 10 minute comparison shopping for sponges is moot.
Google Shopping makes it easy to do comparison shopping. And yet Amazon doesn't want to be on Google Shopping -- I wonder why...
Juggling logins and shopping carts? Heh. Nice fallacious argument.
So here's the thing: It actually does cost $8-10 to ship most of the types of things you can buy on Amazon. I don't know what Amazon pays for shipping, but I would be surprised if FedEx/UPS/OnTrac will even look at an Amazon box for less than $3-5 (to say nothing of transporting it).
Amazon Prime is predicated on the idea that you'll make Amazon your default retailer after paying for prime.
Being a customer's default retailer is a powerful position for two reasons. First, many customers don't shop around for the lowest price on each item (they shop for lowest basket price), so your margins don't get squeezed quite as much. Second, you don't need to advertise (as much) to these people, so it reduces your marketing spend.
As loyalty programs go, it's a very clever one.
So reverse Costco by mail? (Costco makes all of its money on memberships; margins are fixed at a low rate on all products. The loyalty factor is still there though)
I remember ordering from catalogs in the pre-internet era where you would pay $20 on a $100 order for 2-4 week shipping.
Being able to click a button and have whatever I desire show up on the doorstep within two days for $100/year is the next best thing to a Star Trek replicator.
https://en.wikipedia.org/wiki/Fallacy_of_the_single_cause
You don't have to pay $100/year for stuff to show up on your doorstep quickly, compared to the 2-4 weeks you cite.
Compared to your 2-4 week yardstick, the shipping you can get for free is also very quick, usually 2 days, sometimes 4-5 days. And it can be 1 day, which is faster than Prime, if you pay a small (way smaller than $100) $n fee as needed.
In real life, I've never had to pay for 1-day shipping. So that's $n * 0 = $0 that I've paid, all while getting stuff amazingly quickly compared to those pre-Internet catalog orders.
Totally not true. Amazon had a free shipping option for years before Prime. You just had to select it, and sometimes you had to buy $35 or more, same as it is today.