You can enter your discount code at checkout, after clicking on "Proceed" to confirm the products you have placed in your cart. Below the list of products you will find the field where you can enter the code:

Once you have entered the discount code, click on "Apply": you will see that the amount of the discount code will be subtracted from the order total. If you do not click "Apply", the discount code will not be used and you will be able to use it in the future.
It is not possible to add discount codes and promotions once the order is completed.
Individual discount codes may have particular conditions: for example, they may have a limited duration, or require a minimum order amount, and can never be combined with one another. Furthermore, discount codes cannot be used on some special products, such as the Ah...Vengo Calendar. Please carefully check the conditions of the code before using it!