When you redeem Certificates or eCredits at delta.com there is no transaction fee. You can also redeem them at any Delta ticketing location, but additional fees may apply.
This Delta eCerts Credit Voucher will not be honored when purchasing Prepaid Tickets, gift certificates or excess baggage charges, payments on account or Delta eCerts Agency Discounts. Once ticketed, penalties may apply for changes, per the fare rule purchased.
To request a refund for a refundable or unrestricted eTicket purchased with a credit card, call Delta Reservation Sales at 800-847-0578 within the U.S. or Canada. For all other countries, see worldwide reservations numbers to locate a number in your area.
So the airline has offered you a voucher, which is perfectly legal, but you'd rather just have the cash. You'll need to proactively ask your airline for a cash refund in writing. The same goes for if you've booked through an online travel agency like Expedia. They are also covered under DoT rules.
The amount of your flight refund is always the cost of the ticket. The amount of your compensation depends majorly on the travel distance of your flight. In the case of a replacement flight, it also depends on the flight time. It is possible to receive both flight refund and compensation at the same time.
Canceling a nonrefundable fare will result in an eCredit for the full amount paid (minus any cancellation fees, depending on fare type and route). However, if you cancel a refundable ticket, you'll get the amount paid refunded back to your original form of payment.