If you have a discount code, you can use it at the end of the order (before finishing the payment). And you have two options to apply it: In the first step to finalize your order you will see a drop-down with the summary of the order where you will find a section to put the discount code; or before filling in the payment details, the box to put the discount code will appear again. You will always have to check the "use" button for it to be applied correctly.