You get an email and you remember you wanted to add something, and you go ahead and buy it.
That and we have a subscription box from Meowbox for them as well. We used to get treats, but they didn't like the majority of them so it's just toys now.
Personally, I'd like to see them move to a model where it's completely free until you have 100 sales, then you have to upgrade or something. That would encourage more people to use/try it, plus give them a chance to actually see success and start their small business, rather than fail because they're running out of money for their business.
It’s a fine line.
I use it for toiletries and other routine consumables at home. Amazon trucks are going to be on the road anyway; may as well save my car trips for actually interesting trips.
As a real example: my subscriptions are on a 2 month cycle, and get delivered roughly on the 3rd, which gives me until the 26th of the preceding month to make changes. On the 24th they send an email with all the prices that will be charged if I don't change anything. With almost no friction, I can change or cancel anything up until the 26th.
You have to be very disciplined to get as efficient with Amazon. It's probably not really possible.
I get pet food on subscription. Chewy sends me a notice my order is about to ship. I can skip or cancel if I want.
Shopify on the other hand is hostile. I have a small store there I've deprioritized. They have a $9 / mo inactive plan, where they keep your stuff but customers can't buy. If you want to stop paying, they delete all of your stuff & domain: https://help.shopify.com/en/manual/your-account/pause-deacti...