Personally, the silliness in so many of the cover letters makes me dislike them. Since we don't know how the HR people where you will be applying feel about them though, let's assume that they like cover letters. Let's also assume that they don't like silly ones. "I've heard all about your wonderful company and it's my dream to work there with you! I know I have all of the qualifications and attributes you're looking for. I'm the perfect fit for your needs!" ARRRRGH! Now that's what I'm talking about when I refer to 'silly' cover letters. If you aren't sure about yours, read it out loud and pretend like you are the HR person reading it. You have 400 resumes, many of them being screened out due to the silliness in them. If reading it out loud in a dramatic voice makes you nauseous, work on it! :)
So, what SHOULD go in that cover letter. You're very close. I would drop the "graduate of..." because (1) that will be on the resume and (2) you are applying on your professional coding credentials, not on the basis of having taken a course.
I would probably go with something like this. It doesn't limit you in any way, by saying "part-time" or "orthopedic coding position" or "outpatient", or anything else. You can always limit yourself later if they offer you something you don't want. Right now, you just want an offer. Use the term THEY use in their job announcement. Keep your version as close to what they say they want as possible. They want a medical coder? You are applying for that job they have open for a medical coder. They announced an opening for a medical coding specialist? You are applying for the job recently announced for a medical coding specialist. You really don't need to say anything else other than how they can contact you. Don't give them any reason to reject you from the cover letter.
Dear Ms. ______:
I am interested in the position you have open for a medical coding specialist. My coding credentials include passing the CPC exam, through AAPC, and the CCS, through AHIMA. I may be reached by phone at (###) ###-#### or by e-mail at ---@---.
Thank you for your consideration of my application.
Name, CPC-A, CCS
Notice that no matter how dramatically you read the above version, you can't make it sound very silly. It is what it is, a request for them to consider your application. If the HR person is very, very sick and tired of all of the drama (I'm the greatest coder you've ever met!) kind of e-mail and reads them all out loud in a dramatic voice, YOURS PASSES THE 'DRAMATIC VOICE READING TEST' and you will still be in the stack to be considered. The next one she looks at, the one that says, "OHHH how I hope you will allow me to show you what I can do! Please give me a chance to prove myself!---signed Yours Truly and Hopefully" will probably not make the cut. Neither will the one with all the cute fonts, or one that I'll never forget, written on lined, perfumed lavender notepaper. Keep it simple.