UPDATE ON MAIL-OUT
We ended up sending out about 20 coupon/letters (later than we wanted). The response has been quite close to what I expected. So far, 5 responses came in the day they hit the mailboxes . . . 4 were astonished that there were any issues, were impressed we sent out the letters. 2 refused to use the offer, 1 agreed to half the offer, and one was excited to get the coupon since money was tight (she used it right away).
The orders placed by responding people have been higher than the original total, and easily covered the food cost of the $10 off. I expect to get more responses in the coming weekend. There is a 12/31 deadline, so some could come as late as the Christmas holidays.