If you're just gonna put it in a secondary container, why not just package it in jugs? Why can't you just be normal, Canada?
I'd say using plastic bags to hold milk will produce less waste volume than using milk cartons. The world is addicted to plastic because it has very desirable properties as packaging (light and strong) and is obscenely cheap. All these savings may provide a competitive advantage by being able to sell the milk at a cheaper price.