How do I offer customers to buy relevant products after they click Checkout?

In this app you can create informative messages of sales at your store. If you want to actually run the sale, like offering relevant products, you can do it in a few clicks using Upsell For Products - Buy X Get Y app:

In one window, select the target products from your catalog after purchasing which the customer will see the relevant offer
In the other window, select the products from your catalog the customer will see when they click checkout.
In this app (Free Shipping & Hello Bar), create the informative message so that the customer knows about this sale.