Our next goal is to have multiple cashiers, so we can serve multiple customers at ones.
Because this is kind of a big step, we will break it down to two smaller exercises.

The first step is to put all the variables for the cashier-state-machine into a struct and
pull out the code into a function (let's call it step). This kind of code change, which doesn't
change the behavior of your program, is called refactoring.

This may seem like a lot of busy work, but it is really important. Specially if you want to reuse code.

Don't be discouraged, if you get a ton of error-messages. This is normal for this kind of work.
Just read them carefully one by one and fix them.

If this seems to you as to much code change at ones, just take your time and do one TODO at the time. 

Your task is to refactor the cashier code.