You got a $7 meal voucher? Lucky, I only got a $6 one for the last delayed flight I had!
I would expect to receive a compensation voucher in the mail in the next month or so. For mechanical delays of that length, I believe the airline is required to reimburse the customer a certain amount of money. The last mechanical delay I took (about six hours) netted me a $125 voucher in the mail about a month later. I didn't write to Delta at all; it was automatically sent to me.