This may have something to do with the minimum wage increase in California along with other rights of an IC. It also could be regarding terms of when work is completed and someone emails you saying you are not getting paid when you did this, when you did do the shop correctly and you are penalized with no pay without them giving you a chance to correct.
It could also have to do with them saying if you don't respond to this email asap you will not be paid. That breaks labor laws. They also email like insanely up to 40 times per day- same shops same area. Its annoying. It could also be that the $10 they pay for some food reimbursement shops add up to more than an hour with the admin time and that breaks the minimum wage law it certain parts of California.
Many of these companies are on thin ice when it comes to this. Not paying shoppers. I would check directly with the Labor Commissioner Office on the phone before you send out your SS# etc in that letter received to make sure it is legit.
Also some of these companies are not paying on time and you have to email them for late payments. That in itself it a big violation.