I don't think DDD gives anyone credit.
I can also vouch for DDD, since I've also spent several hundred dollars there over the years. Only once did an order get lost in the mail. I called them up and I got a replacement in a week. The free shipping is slow since it's media mail. It usually takes a week to get to me from Chicago, and I live in Iowa.
Twice a year DDD has a 20% off sale (one in June, the other in November) but I doubt they have any coupons now.