This article explains how to apply a valid coupon to your plug-in purchase.
Viewing Your Available Coupons
Available coupons are displayed on the shopping cart page. Click the shopping cart icon in the top right corner of the page, then click "View Cart" to proceed.
Applying a Coupon
Copy the desired Promo Code. Be sure to only copy the bold text with no preceding spaces. Paste the code into the box labelled "ENTER COUPON CODE" and then click "Apply." If the coupon code is accepted then a confirmation will be displayed at the top of the page and the total will reflect the discount.