Whoa dude. First database mailer with a huge offer? What were you thinking? Something like that is fine for a month like February, but in December? You are going to get your as handed to you, and not in a good way.
I am sure you will learn from experience here. Just staff up, prep, up and prepare to get your doors blown off. You might want to print up apology letters or $3 off post cards now to cope with the service complaints.
On average, I see first time mailers (any mailers, $3 off or free side menu item) pull in the 30% range. A $10 gift cert is going to pull stronger. If you can, delay as many of those mailers as you can until the beginning of the year. Jan is a slower month, and that kind of aggressive marketing will really help your sales. Sure, packing it into your biggest month will increase sales incrementally, but at what cost to service?