Ummm, I have Tesco slots booked once a week for the next three weeks. You can have multiple bookings and of course you can cancel or amend up until the day before delivery.
we used it when they first started doing it because that was the only way we could get on line shopping at the time as we had never done it before the supermarkets were not taking new customers , Morrisons from Amazon took the order at 10am and it came 2 hours later