It seems they can be used online. Also they don't expire. From Delta's website -
Certificates and Cards can only be redeemed using one of the methods specified below:
- Go to www.delta.com/redeem.
- Call Delta Reservations and advise the agent that you are redeeming a Certificate or a Card. In the U.S. and Canada, call 800-221-1212 or 800-225-2525; in Japan, call 0570-077733 or 0476-31-8000; and in all other countries, call the local Delta Reservations office. This method of redemption may result in direct ticketing charges.
- Call Delta Vacations at 800-800-1504. (Travel agents, call 800-727-1111.)
- In person at a Delta City Ticket Office (CTO) or with Delta Airport Customer Service (ACS).
- Through a Delta General Sales Agent (GSA).