Well, you probably don't mean you can set up a stationary camera as if so then you could just install some sensor at the door or a foot pad. But if you could m

Well, you probably don't mean you can set up a stationary camera as if so then you could just install some sensor at the door or a foot pad. But if you could mount a phone... Maybe some technique where you take a baseline photo with the store empty and then compare that to ones with people in it. You'd have to account for natural changes in light and ignore that in the algorithm. Shadows and bags would be difficult. Then count the number of changed pixels from baseline, the average number of pixels representing a person, and some sort of edge detection for added checking. Or, a mechanical turk worker would solve this in a few seconds for probably a penny.