After you have selected the quantity of tickets, the next page will show you something similar like the following message;

 

 

and the next page will provide you with a box to enter any offer / voucher codes you have;

 

Please ensure that you are entering valid codes and these codes are to be entered before checking out. Please check that your order is correct before proceeding with the booking as mistakes when ordering cannot always be corrected and any corrections are discretionary.