"Receive a full refund of the Instacart Express subscription fee immediately"

They have an option to do that - but if they take no action, they wont receive a refund. And that only applies to the 71 users effected by the bug. The people who got the email but took no action evidently can't even claim a refund.

Can you imagine the backlash if Apple started charging $99/year for iCloud and auto-billed everybody who didn't cancel the service?

There's the other issue of the notice itself. If they decided in March to make the change, how much time did they give customers to decide? Given the timeline, it seems like it was "effective immediately" rather than giving proper ahead of time notice.

Nonetheless, even if they did discuss refunding, the entire process seems shifty. Every service I use (I checked!) warns me five business days in advance by email that they are charging my card or sends a physical mail two weeks in advance. Instacart should be sending an email on each event with the delivery fee spelled out clearly.