* Whole Foods (for delivery) releases slots at midnight. This is pretty obvious, because you'll actually get what is basically a, "rate limiting" dialog page if you don't start checkout successfully. The site is getting slammed THAT hard. So for 2-3 minutes, hope you actually can get to the page to confirm placing the order. Those slots go FAST (< 5 minutes).
* Amazon Fresh seems to start releasing slots at 12:30AM. These seem fairly easy to get at release. No rate-limiting page for checkout.
* Instacart seems to have a "semi-hold" for orders in carts that aren't yet confirmed orders. I can watch delivery slots for the next 2 weeks disappear until none remain. But at midnight, they become wide open as soon as 3-4 days out. If you're on point, you can get one early in the day, where stock is likely good, or late in the day, where places like ALDI will have replenished the shelves since the rush is before noon.
Amazon (Whole Foods and Fresh) seem to only hold items in a cart so long if you've not purchased. I've not nailed it down yet, but it's measured in "hours" (not sure of the number), not days. My recommendation is to have a firm list and start filling your cart 60-90 minutes before the above times. That seems to get everything committed in the order 100% of the time.
If you're trying to stock up to not have to go out for a good while, consider a, "defense in depth" strategy. Place a Whole Foods order, place a Fresh order, place an Instacart order. Stagger them by a few days. If something comes in the first two orders, remove it from the Instacart order. You may be able to cancel the Instacart order several days in advance, and still have gotten basically everything.
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I'm done buying groceries online for a long while, so rather someone who could use it have the knowledge I learned through far too much effort trying to figure out how the black boxes worked. Legitimately felt like I was routing a speedrun for a grocery video game to get a world record.
My mother-in-law's strategy for getting a slot with Wal-Mart grocery pickup has been to pre-fill her in-app cart in the evening, and set an alarm to wake up a few minutes before the window to place the order and get a pickup slot opens up (I think somewhere in the 12:00-2:00am timeframe). This may be applicable for placing Instacart orders (and the like) as well, unless they require you to pick an available slot before populating your cart...
We haven't tried at very odd hours as you suggest; we'll give it a try, thank you.