If your package contained a print voucher, eg; £10, then you would enter this code during the checkout process.
For instance, say you liked 3 photos from the session. Simply click "BUY" on each of them to add them to the shopping cart, then when you're ready proceed to the checkout. During the checkout process, lookout for :
Enter the code you were given and click APPLY. The cost of your prints will now be reduced by the value of the coupon.