Much easier to just drink enough so there is no chance of that happening.
But then I am in UK where milk is easily obtained in 2 pint or less packages and is all long term - over a week. It is harder to gat 4 int or gallon containers which I think are more common in the US.
>In other words, the 1/2-gallon container, buy two of those, and it costs significantly more than a single 1-gallon container.
Except sometimes the 1/2 gallons will be randomly on sale where you can get like 3 of them for the price of a gallon. Milk economics makes no sense to me. But yeah, it's usually cheaper to buy more than you need and just throw it out if you don't use it, as is the American way.
In the US, the way milk is sold, is that larger amounts cost less. In other words, the 1/2-gallon container, buy two of those, and it costs significantly more than a single 1-gallon container. It gets even worse for quarts. But I seldom buy in the 1-gallon container as it will generally spoil before I've used it all, so there isn't any savings there for me.