In this app you can create informative messages of sales at your store. If you want to actually run the sale, like BOGO (Buy 1 get 1 free), Buy 5 get 2 free, etc., you can do it using Upsell For Products - Buy X Get Y app:
In one window, select the target products from your catalog, after purchasing which a customer will see the pop-up with the Free Gift offer;
In the other window, select the product with the price $0 from your catalog (you’ll need to create this free product in advance). The customer will see this free offer the popup when they click Checkout.
In this app (Free Shipping & Hello Bar), create the informative message so that the customer knows about this sale.