The free super-saving shipping can be tedious. I think if things are located in multiple warehouses, amazon first sends everything to one warehouse and then packages up your order and ships it. That’s why it takes a week in some cases. I guess it comes down to logistics and money saving for Amazon.
Amy and I use Amazon Prime which gives us free 2-day shipping on pretty much everything Amazon sells. They charge $70 a year for it, but if you order a lot of things during the year it can save you a bundle, both wait time and money. A couple years ago I had to order a bunch of heavy tools (air compressor, nail gun, saw horses, radial saw, etc.) I got it all shipped via amazon prime in 2 days. My only regret is Amazon makes it easy to forget the smaller local stores that prop up the local economy. So we try to use it within reason.
Of course, the Amazon Prime feature does make one a bit more of an impulse shopper. I am sure Amazon figured this out a long time ago.