Summary: Allow users to reserve delivery windows before they start shopping; clearly communicate delivery minimums and fees; allow users to specify substitutions for low-stock items as they shop.
In the first days of the COVID-19 pandemic, online grocery stores in the US were overloaded and often failed to meet the sudden demand. People were desperately filling in their carts only to discover that their order could not be delivered or there were no pickup windows available. Those lucky enough to place the order found that many of their items were missing upon delivery. Grocery stores were overwhelmed, the supply was low (as people were stocking up), and there were too few employees available to deliver the order or help with pickup.