Go to the post office and tell the person at the counter what it is you want to do. They will show you what forms you will need and how to fill them out. It's easy and it will give you the proof you will need to show that they received your letter. Heaven forbid, if they go against your wishes, you can nail them with this proof. It will cost you a little extra for the signature, but it will be well worth it. Not only that, but they will be signing for the letter before they know what it is. Once they read your letter, it should put the fear of God in them knowing you have this proof.