There is a button in the cash for this, though im not sure about the exact price. It's possible that at times youre being charged just for a cup of milk, and sometimes for a cup of milk with the extra charge of a milk substitute (+80cents) because it's coconut.

There are two rows of milk substitutes you can choose, one side will charge extra (use this for drinks made with milk like a latte, or a cup of milk) and the other side is just for a small add in of milk with no charge (like adding 1 milk to a coffee).

Chances are, the most expensive one you've paid was the right price. A cup of milk with the extra charge of a milk substitute.

Where I live, a tall cup of just milk is about $2.50 if you order it as a “tall milk” regardless of whether it dairy or nondairy. That equates to 12 oz

If you are ordering other stuff and ask for a cup of milk “on the side” they may ring it up differently or not at all depending on the store and the barista.

They change the prices on our machines without our intervention and without notice to us or our input. So that can account for price changes, updates that we have no control over. Or maybe one time you didn't get the upcharge for a nondairy milk

I don’t know the price off the top of my head, but they should be ringing you up for a tall “cold milk”, and then using the button that charges to modify the milk since it’s more than the free “splash amount” modifier. Depending on where you live the prices might be different, but for a tall I’d assume it’s somewhere around $2.45 for a tall cold milk, and then it’s .60 cents for the alternative milk upcharge, so it should be just over $3.