Abstract: Systems, computer-readable instructions, and methods for storing products are disclosed. For example, the system may include modular shelving units that include a shelf which is placeable on a rack, the shelf including a platform, the shelf having a weight sensor below the platform, the platform is configured to support N number of holders, each of the N holders configured to store products, each shelf further having N?1 optical sensors below the N?1 holders when the N?1 holders are supported by the platform and N?1 windows above the N?1 optical sensors, the N?1 optical sensors being in proximity of the N?1 holders, respectively, wherein each holder includes an opening, wherein when a holder is on the shelf, the opening is capable of being aligned with an optical sensor, the shelf further having a processing circuit coupled to the N?1 optical sensors and the weight sensor.