M M.

I've used Instacart once - yesterday - and haven't decided if I'll use them again, but many of the issues cited by customers below are avoidable.
1. The members fee is if you sign up for their Express service which provides free delivery. It states on the website that Express membership is $149 / year.
2. The service fee is optional. It does not go directly to the shoppers, and probably is a slush fund for the company, but you don't need to contribute. You can change or zero-out the amount during checkout. I learned this the hard way, but their customer service was happy to remove the fee for me after the fact.
3. The shopper you get is a total crap-shoot. Mine was good, it seems others may not be. Part of the problem is the bad website. For Fairway there were a lot of items listed that my nearby store did not have. Luckily my shopper sent replacement options in a timely manner. Maybe she missed some things that were there, but on a bill of $180 there were three replacements (that I approved), 2 totally unavailable, and 1 mistake. So even if it was the shopper's fault, I can hardly complain given that 25 or so other products were done correctly and arrived on time.
4. The service fee does NOT cover the delivery people, who are different from the shoppers. Please tip directly for delivery!
5. Customer service has been great to me -- perhaps I've just gotten lucky. They refunded fees that they didn't need to, and they are delivering to me a single loaf of bread that was mistakenly replaced with another similar one (the shopper's mistake).
As I said, I may not use them again. There are problems like not being able to get less than a whole lb. of lunchmeat, or getting exact amounts of produce one might like. But, on a very rainy, hot day when walking 28 blocks with my granny cart was an unwelcome idea, they did a pretty good job.