You are not limited to simply creating buttons for products, you can also create a discount button and a link button in the cash register for the cashiers usage.
These instructions are for Backoffice but you can also create buttons directly in the cash register by turning on the edit mode in the bottom left corner if you are logged in as an administrator.
Discount button
Start in the button map you want to add the buttons to (Products -> Button Layouts).
Go into the button group you want to create the button in.
Click Edit Mode.
Click an empty spot, marked with a plus button.
Choose Add Tab Discount as the Type.
Pick the Discount Type, Percentage or Amount.
Enter the discount.
Enter a name for the button if you want.
Change the color of the button if you want.
Click Add.
You can now Save Changes at the bottom of the page if you are satisfied.
Remember you will need to log out and back into the cash register for it to update.
Link button
Start in the button map you want to add the buttons to (Products -> Button Layouts).
Go into the button group you want to create the button in.
Click Edit Mode.
Click an empty spot, marked with a plus button.
Choose Open in browser as the Type.
Enter the link that should be opened.
Enter a name for the button if you want.
Change the color of the button if you want.
Click Add.
You can now Save Changes at the bottom of the page if you are satisfied.
Remember you will need to log out and back into the cash register for it to update.