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.