Even if you weren't worried about the environmental (or other) impact of throwing out food, there is clearly a non-zero cost associated with acquiring a second

Even if you weren't worried about the environmental (or other) impact of throwing out food, there is clearly a non-zero cost associated with acquiring a second gallon which you will not drink. You have to carry more stuff home, keep more stuff in your refrigerator, worry about and monitor the state of the milk in the refrigerator, and throw away the milk once it's bad. It is very clearly a worse deal in every way.