This information comes from WMATA.com.
The most efficient way to register your card is to add it to your WMATA online account. You can also fax a note with the SmarTrip® card serial number, your name, address, and email address to 202-962-1035 or mail the same information to:
SmarTrip Regional Customer Service Center
PO Box 220568
Chantilly, VA 20153
Note that if we mail a new card to you (e.g., to replace a lost card), we automatically register the card on your behalf. However, if you have an online account, we recommend that you also add the card to your account.