To mark customers as paid/unpaid on the tee sheet, the first step is to enable the employee role to be able to do so!
Only Owners have the ability to edit/modify employees and their roles.
To enable this feature, go to Settings > Roles and click on the role you wish to give this permission to, and turn on Can mark products in reservations as paid/unpaid
Once turned on, by navigating to the tee sheet, click to open a booking.
Next to the price of an item in the booking, if the item is not showing paid, click the little circle next to the item, and then select "mark as paid" which will highlight the item as paid with a green $ sign.
To mark customers as unpaid is the same action. Open the booking, click the green $ sign, and select "mark as unpaid" which will remove the $ sign from the product you choose.