Micro Pressure Sensors for arduino?
Pressure sensors exist, and also in MEMS technology, that means that the average dimension is 5x5x0.5 mm or less. But maybe (depending on the weight of the boxes) you may need strain gauges, and then the thing becomes complicated, as you need a bridge circuit and a very good differential amplification system.

Even using pressure sensors, each pressure sensor costs usually around 1$ (roughly), and building a 100x100grid would mean 10k sensors (and some thousand $). Do you really need this resolution? If the package is solid, you may need only 4 sensors at the corners, or maybe a little more, but still less than 10k.

Another consideration: MEMS sensors are mainly for PCB mounting, so you'd have to build a structure with these sensors soldered, and wiring a good interface depending on which one the sensor uses.