I've done dozens of these, it's the easiest money I've made doing shops. It only takes 10 minutes tops to do. If I were you I would get as many of these shops that you can get your hands on. You don't need a letter, just stop at the service desk and sign in.
I'm curious, can you say what company it is? If it's a different company I want to see if I can get some shops from them.
Thanks