Use U.S. Post Office Certified Mail to get a tracking number.
Registered mail or return receipt requested is not necessary. We tested using return receipt requested vs. not using it. The Post Office web site showed that all our packages arrived. They were not returned. However, not once did we get a return receipt, so don’t bother with that.
You can use the Click Yours system to enter the tracking number in advance. Or you can write it on the cover letter. Or you can write it on a small piece of paper, and paper clip it on top of the set.