Running an e-commerce store using WordPress is going to be much easier if you use WooCommerce to power your website. WooCommerce is not only powerful, but it also has plenty of awesome extensions to help you improve your store. Write a Review Reminder is a simple plugin that lets you remind your customers to leave feedback for your products.
The plugin sends an e-mail reminder to your customers to remind them to leave a review. Upon approval of each review, your customers receive a thank-you coupon. You can manage coupon settings from the back-end. This is a commercial plugin.