Pay-per-click users typically go straight to product page and therefore, don't see any special offers or discounts. I suggest a right-side-of-screen column with Admin view-controllable panels, one of which should show information and/or links on specials and discounts.
In our case, we offer free shipping for orders >$100, but customers don't see it unless they visit the homepage.
Great sites make the most of visitors and add-on sales are easier than the initial sale trigger. NopCommerce needs a lot of improvement in features that promote add-on sales.
Some ideas
1. Pop-up window that appears after an item is added to cart (e.g. "free shipping on orders > $100").
2. Right side panel that appears on all pages - cart summary, specials, etc.
3. After purchase special shopping opportunity (unique discount code generator that expires within a certain amount of time) - "Send a gift and save money! Use unique discount code XYZ by midnight tonight to save X%"
4. Register to receive a discount on this or next order.
