This is more of an issue with instacart. The vast majority of online groceries in the U.K. are served from supermarkets by supermarkets and the fulfilment rate is 95%+ with decent substitutions (typically a free upgrade to a more ‘premium’ product that you can decline at the doorstep)

Can you expand on what you mean by some random subset of what you ordered?

If you do these online orders from retail stores, a lot of times they just bring you some of what you order and then tell you the rest is "unavailable". But you can't cancel the order because they already picked it for you.

You order a hammer and a saw, one store has the hammer, another the saw. They are one order but may arrive different days and it’s not always well coördinated or communicated with customer.

I think they meant "random subset of what you can order"

No, they mean random subset of what you tried to order. If you use Instacart, at least in California, there is a good chance that 1/3 of your items will be "unavailable" and they just won't get it.

The problem is, if you order flour, milk, and sugar, and then they don't bring the sugar, you can't make the thing you wanted to make. But you also can't return the partial order.

Got it, that makes sense.

And that's legal? The more you know...

Speaking as a California scrooge, I still prefer it to the alternatives honestly. My neighborhood has one mini-target and three local grocers one of which is a co-op. All three local grocers are garbage, their quality is fine but nothing wild and most of the price is for pretty looking food that doesn't scare the local yoga moms. Also, because they are all small and the target had to agree not to sell certain things the food variety available is not stellar. Thanks to Instacart I can order from places with real prices and a wide range of grocery products. Instacart is especially great because Amazon Fresh, although better than the local options for variety, is also wildly over priced and so if I want delivery+thrift Instacart is basically the only game in town because the bar is that low. I am thrilled at the idea that some of these local shops might finally have to adapt thanks to the increasing presence of delivery services.

TLDR: I think in house shopping services are your best bet and they will only get better as companies invest more and more in them. These will hopefully replace Instacart.

I have ordered all of my groceries since March using various online services because my wife and I are both high risk for covid (asthma + pregnant). This includes Instacart, Whole Foods Delivery, Whole Foods pickup, and now Stop and Shop Pickup (effectively Peapod delivery).

Instacart is by far the worst in terms of reliability. Sometimes the substitutions make absolutely no sense whatsoever. The integration with the stores just aren't good enough and it relies too much on the individual shopper to to pick something reasonable it seems. I have had good shoppers and I have bad shoppers that take too much leeway. I don't blame them though, its just what it is. In my experience the item catalog for my local stores is also inconsistent too. Items with the same name at different prices for example. What am I buying?

The most reliable system for me is Whole Foods pickup. I have to go to a whole foods that is about ~20 minutes away instead of 10 minutes away, but the pickup basically has unlimited slots. I have never not been able to pick any slot I want, even same day. Delivery is much tougher to get slots, so I don't even try anymore. Whole Foods lets you select the actual item you want them to substitute with, although they only give you the closest ~5 options, which sometimes aren't perfect. For example, if you are trying to get lactose free yogurt, not all the replacement options will be lactose free. You also can't talk to the shopper, you can only approve or deny substitutions when you leave the item on "substitute best available". The cons basically come down to selection and price. The selection seems a lot more limited than in store which can be annoying. Prices are obviously Whole Foods prices. Another unfortunate thing is that you can't tip the shoppers. I tried to leave money in my trunk for them to take, but its against the rules I guess, so some would take it and some wouldn't. I ended up stopping because I didn't want to put people in a position where they they could get in trouble. You can tip on delivery and I always assumed that some of that tip went to the shopper, but I guess not.

Stop and Shop pickup is a close second. It's run by their Peapod service, but you just pickup curbside instead of waiting for delivery slot. Much better selection and prices, but there is no way to pick substitutions, you can only say "Do not substitute this item". I will probably use this more often going forward to supplement my Whole Foods orders. I can hit both stores for pickup in less than hour. The people that do the shopping and delivery curbside can accept tips.