Ocado are wonderful. I start to assemble my order about 3 weeks before I need it (you can go back and edit the order at any time) to take advantage of special offers - any item that you order when it's on offer comes at that price even if the offer has ended by the delivery date
There are very few substitutions, and those are usually for something better (last Christmas I ordered 4 books of second class stamps, and they were substituted for 4 books of first class instead - and they were large books, with 12 stamps in each..!)
Forgetting where you left your keys does not mean you are “losing it”