Legal Question in Business Law in California

It depends on the store's policy. However, putting merchandise in one's own bags probably appears to be suspicious activity, so I would not encourage doing that lest one is detained on suspicion of shoplifting.

There is no law about this. Stores are private property, and are free to make their own rules about what customers are allowed to do. But as Mr. Cohen notes, putting merchandise into your own bag will look suspicious. And if you still have things in your bag after you put your purchases down on the checkout counter, they store may believe you're hiding other merchandise in the bag. They would be allowed to detain you if that happens and, perhaps, search your bag.