What do you think about a radar approach like Google's Project Soli?
I was thinking more in terms of lidar tracking of people. It's been around for nearly a decade and works quite well.
Distance - I'd imagine that the distance from the top of a door frame to the top of the head of the average shopper is roughly equivalent to the distance from the Soli device to someone's hands.
Software - The Soli hardware streams radar contacts to software trained to separate and classify them, not too unlike the approach that Density is already using.
Benefits over a CV approach might come in power usage and lower complexity in the contact classification software.