Yes, what you want to do is go to your local post office (or you can order them online for free) and grab a handful of Certified Mail slips and Read Receipt cards (if you are going to need them for OCs/CAs). They don't cost anything.
That way you can fill out the CM slip in advance. Get the number off of the slip and enter it into your letter. That way you definitely have proof that your letter was sent CM.
