Printing out a copy of the PDF Invoice/Receipt that the system generates is quite straight forward.
Simply click in the booking in question.
And then click on the Invoice/Receipt button at the top of the booking details panel. This button will say 'Invoice' if the booking has an outstanding balance or 'Receipt' if the booking is fully paid.
Once you click on the button you will see the PDF version of the Invoice/Receipt, you can print this off if required.
The icon highlighted in red below is an optional setting that can be turned on the lets you print out a receipt from a thermal printer - to print off a small credit-card type receipt.