I think to be fair those people who weren't able to go to the shops didn't realise how low supplies actually were, and how long you had to queue to get in and get round.
True, but I think most people realise that expecting somebody doing you a favour - somebody who already works 11-hour days - to go to four different shops to buy what can be bought in just one of them will take them an unnecessary long time for not a great saving, covid shortages or not.
If you're one of these people who makes it into a hobby to spend all day trawling around multiple shops finding the cheapest price for every item, then good luck to you. It's a conscious personal decision you make that the small financial savings are worth more to you than all of the extra time you spend travelling to, shopping and queuing up to pay in lots of shops (unless you are so very hard up that you genuinely have no option but to do so) - and that's absolutely a valid choice to make FOR YOURSELF.
If somebody else - especially somebody obviously with far less spare time than you - is putting themselves out and doing it on your behalf, you graciously thank them and repay them what they spent on your behalf. In fact, I think most reasonable people being done that favour would cheerfully automatically round it up by a few pounds by way of thanks and to contribute to petrol and parking costs.
To deliberately withhold some of the money actually spent on the items that you requested - from the previously agreed shop - is not even cheeky, it's theft. My simple response, as in all these scenarios, to people who protest that they could have done something cheaper/more quickly/better themselves is 'well, do it yourself, then'.