Which would you rather do, Tweet, or message at the point of purchase?
Not that the two are mutually exclusive, but hey, if you or your marketing people have figured out that somehow a sales or promo message is better on a phone than on your storefront window, then by all means, tweet.
On the other hand, you might conclude that it is better to message to the customer when they are right in front of your store, and that creating a way for them to look more closely at your window and tell them to come inside and get 10% off anything in the store, is a more effective investment of your marketing dollars.
You may have plenty of money for either investment, but then again, maybe not. If not, invest in your own location and make it as powerfully attractive as possible. This is the entire premise of Window Video Systems, by Looknglas. Put a bright, moving, motivating message in your window. And then refresh it whenever you want. Or improve it. Or add to it. Or put accompanying product images along with. As many as you want.
Can't do that with texting, can't do that in a tweet. Do the smart thing. Get a Window Video System.