From what I’ve read, you are better off sending your letters via certified mail with return receipt. This will provide a paper trail in case you need to dispute or keep records of your negotiation. I have sent some certified, but not all.
Also, try not to copy the letters you see online. Use them just to have an idea of what you are going to say, but in your own words. Make it short and get to the point. I’ve had better results this way.