Regular milk — one of the worst offenders when it comes to spoiling easily — once opened, usually only has a refrigerator life of around five to seven days, and even less for whole milk. So unless you have a very large family or drink a lot of milk every day, it's likely that a large Costco milk container will go bad before you have time to drink it.

Costco sells gallon jugs of milk in two-packs and the price can seem like quite a good deal at $4.99 for 2 gallons of milk. However, the money you save won't add up to much if you end up throwing most of it away. Moreover, Costco's large milk containers can be hard to pour, making it more likely you'll end up spilling more of your savings on the counter than drinking it.

Instead, Eat This Not That recommends purchasing your milk alternatives at Costco, as they carry a wide variety of plant-based milk, from the classic soy and almond to oat, hemp, coconut, and even pea milk, via Instacart. These don't expire as quickly as dairy does, which makes buying them in bulk a much smarter option.