OrderReviews is an OpenCart extension, which creates and sends automatic emails to customers asking them to review the products they have bought from you. They can write their reviews directly from the email or from their favorite browser. This is the perfect solution to increase the reviews and the unique content on your store. To stimulate that, you can also provide the customers with discount codes which they can later use on your site. To put it short, the module has lots of options and you will have to find out which configuration works best for you.