Instacart delivery workers are classified as independent contractors, which allows the company to flout basic labor laws. This is commonplace in digital age 'gig economy' jobs, but dates back to the early days of food delivery - e.g., most pizza delivery drivers are classified as independent contractors.

That having been said, the concept of defining contractor wages in relation to customer tips is new to me. I could see a legal argument being made in the employer's favor if the worker gave due consent to the transaction.

The idea here would be: Instacart states somewhere on the order prior to pickup 'if you choose to accept this order, you will receive $10, of which $.80 will come from us.' Since the delivery worker isn't running a 'shift' as an 'employee,' but just coincidentally happens to be running Instacart orders for 10 hours straight, this counts as one of many transactions that they've accepted and hence waived the legal right to complain about.

If this legal fiction sounds absurd to you, you're not alone.