How to populate email field based on the dropdown selection?

How can I auto populate an email address when a name(cashier) in a drop down box is selected?

You could do that using our conditions.

I would recommend checking out the following guide, but if you have any questions along the way we would be happy to assist with them: Smart Forms Using Conditional Logic

As you go through the guide you will see the option to Update a form field value, which would allow you to do this.