Although it does not replace copyrighting your work, it does offer quick protection. I am posting this only as information to help one another.
Most writers are concerned about protecting their work against infringement, and rightfully so.
The main thing you need for protection is proof your work was in your possession first. While copyright registration provides additional legal remedies, there are two less expensive protective methods anyone may use. When you receive your own envelope, don't open it. The certified date on the unopened envelope can be proof of possession as of that date. Either should work well until you register with the copyright office.