I am designing an outdoor vending machine that I need to detect if there is product available for sale in a cell approx 2ft. x 2ft. x 2ft. I'm looking for the best way to detect product . Temperature ratings on this machine could go as low as -40C and as high as 80C depending on where it is located in the world. I have been searching for a cost effective solution of either ultrasonic or IR type sensor module that can operate in this temp range. I need 8 of these sensors per machine hence the cost effectiveness becomes very important. A module that can be coded into an embedded controller is what I'm seeking. Anyone have any experience with these devices?