You can use your promo code if it's valid. You can use only one at a time.
You can add a promo code on the payment page, in the order review on the right-hand side, by clicking on "Have a promo code?".
Promocodes are not applicable for some products, please check T&C point 9 "Terms and Conditions for Promotions and Special Offers"
It cannot be applied to bookings previously made. It can only be used via the online payment method.