Product counter below a tray

I can envision several methods that all will require considerable tinkering on your side:

• Use a camera and a machine vision program to count elements
• Use reflective optical (infrared) sensors, one for each compartment below the tray and a computer to count filled boxes.
• Use mechanical switches instead of optical sensors. Will require some kind of lever to be activated whenever a box is filled.
• If you are interested in the total only, use a scale and weigh the whole tray. Determine the number of filled boxed from the weight of the tray.