How/Why do you sell liquids by the pound?

We sell everything by the pound unless it is a unit unto itself, like a bar of soap, or a prepackaged item like almond milk.  We need to be able to fill each person’s container with, and charge them for, only their desired amount, which we can do by the pound for everything.   The container is weighed in advance, and then the difference with the amount once the content is in is what is charged.