We've done this in the past and while it had excellent sales volume results, there was one unwanted result as well.
Multiple competitors complained to some of the larger manufacturers that we were selling widgets well below standard retail. Even though the sale only lasted two days, we narrowly avoided a price war and frozen wholesale accounts.
It depends on the market, and it depends on your competition. I'd be very careful automatically entering customer coupons/discounts. You never know who's watching.
It might sound petty, especially around the holidays, but it happens.