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...