[xyzzyz]

Losing coldness due to opening door is really not very relevant. Most of the cold in a fridge is stored in the items you keep in the fridge, not in the air itself. A typical fridge holds maybe 1 kg of air inside it, and cooling 1 kg of air by 17 Kelvings is 17 kJ, which is probably a minute of compressor running. Opening a fridge and losing all cold air from it thus costs you probably something like a quarter of a cent.