EdX certificates are delivered online through edx.org. EdX does not offer physical copies of certificates by mail. Once course grades are finalized, if you earned a certificate you will see a link to view this on your dashboard.
Many self-paced courses issue certificates as soon as a student has a passing score.
You can print out your certificate using the printer icon in the upper right when viewing your certificate:
Many operating systems will also allow you to print to PDF if you would like to save an electronic copy to your computer.