they only get to use a couple servings out of their 5 gallon non-dairy milk jug

Don’t buy so much at one time then? Doesn’t seem that difficult a problem to solve, I do it every time my dairy-averse partner stays over for the weekend. Buy an amount that is reasonable for expected usage needs, it’s easy. I’m not out here buying a 5 gallon bucket and then whining when my guests haven’t drank all of it.