Does Costco Sell Single Gallons Of Milk? - Tasting Table
Briefly

In the U.S., half-gallon containers are the most common size; entire gallons are reserved for those who need a hefty amount in storage. However, the issue is that at Costco these larger sizes often come in multipacks, leaving you torn between waste and getting in the car for (what feels like) the fifth time this week.
Customers have reported varying availability across the U.S., with single gallons spotted at stores in Michigan and Chicago, yet only double packs on the shelves in Fairbanks and Los Angeles. It's just the luck of the draw.
The general rule of thumb is that milk lasts between four and seven days once opened if refrigerated at 40 degrees Fahrenheit. Of course, how long milk will last sitting out on the counter is a totally different question.
Read at Tasting Table
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