£2.50 for stuff out of stock is a pain in the arse and really unhelpful especially for people in your situation. That's where it frustrates me with online ordering and out of stock stuff. I can't plan for a week if I can't guarantee I'm not going to get everything I've planned on needing. Tbf though if I have to do a top up shop for a bunch of stuff and have to drive its at a point where spending another £2.50 on delivery isn't bad. If I go in store for a top up I will spend more than I need to and it will cost me petrol. If I'm cynical, I'd say that gaps on shelves drive up sales because people have to do that top up shop - at a time when people have been trying to do meal planning a lot more.
I try and shop on a two or three week basis and rotate supermarkets where I can. There are certain things I know I can get in certain supermarkets for cheaper, so I try and save getting those things for when I buy at that supermarket. For example if I know its 20p cheaper for bacon at one supermarket, its worth me not buying in week one, and instead buying two packs in week two at other supermarket. It helps me plan my cooking too. I can guarantee what I have in better too. You need to keep an eye on prices as they can change but if you have a basic list it's just worth checking every once in a while how things are comparing.
I do find that milk and bread are the one thing that I do need to top up on during the week. I trying to walk to the local shop for those. I know I'm paying over the odds to do that, but if I walk I am less likely to buy a bunch of other stuff cos I can't carry it. And walking means I'm not spending money running the car. Its also good exercise.
I appreciate that this last thing isn't something that's likely to be something relevant to you but I think its worth mentioning for the sake of others.
Its more about illustrating how certain behaviours lead to you buying things you don't need (and the converse). I try not to go to the supermarket without a list - the spontaneous top up shop is my achilles heel because that's when I tend to go off piste. Which is why I've changed my habits and patterns to insulate against shortages.
I don't think the rewards you get from loyalty ultimately add up. The system is rigged to try and get you to do add ons and encourage combo purchasing as they have a team of people who look at behavioural patterns to maximise their profits. If you know some of the supermarket tricks on how they do this and are conscious of the psychology, you can avoid a few of the traps.
As i say, the top up shop is the real killer for me. I prefer to do a double supermarket stop to reduce the need and stop me spending more.