I don't think the quick scan and checking for undercuts go together. Quickscan just gets a list of the cheapest prices, regardless of who they belong too. To get nudercuts you add-on still needs to scan the seperate items for a list of what yours are specifically doing.
That said, I find Auctionator to be VERY helpfull with it's "check for undercuts" function. It's not fast, but it checks all your auctions, and then gives you the option to cancel the ones that weren't the lowest priced. You need to press the button a few times, once for every auction you cancel, cause I don't think it's allowed to automate that in to 1 button click.
The other option is to use Tradeskill master, but personally I find this programm more trouble in setting up than it saves in the end, IF YOU JUST USE IT TO SET AND REFRESH AUCTIONS. Otherwise, for things like glyph producing/selling and such, it's simply the best.
EDIT: Just to add Auctionator does have a quickscan function, it's just not combined with the check for undercuts function.
Personally I'd never use anything else than TradeSkillMaster for posting and cancelling auctions; sure it takes a little time setting it up, but it's so tidy and easy to use once you know how to use it.
TradeSkillMaster no questions asked. It's the gold standard for AH addons, and has evolved though a number of others (auction profit master, quick auctions, and some others I can't remember) to give a very well developed and easy to use product. Once you get through the initial set up (which isn't really that bad), it's a breeze to use and does exactly what you asked for and so much more.
As someone above mentioned, it also has a great crafting module as well as a number of others (the destroying module is a HUGE time saver). Check this addon out for sure.