I have this tip: When I find a slot available I immediately book it and order anything that will fulfil the minimum purchase ( e.g. £40 for Sainsbury’s £60 for Waitrose). That way, I can checkout and secure the slot. Orders can be amended up to the night before delivery, so I don’t have to buy the stuff I ordered. I make my shopping list, then amend the order, deleting the items I don’t want and adding the items I missed.
For example, last Friday I booked a slot with Waitrose for next Friday. At the moment the order is for 6 bottles of wine. Tomorrow, or on Thursday, after I’ve made a shopping list, I’ll amend the order. deleting most of the wine and adding the groceries we need.