ONYXBOX Coupon: Free Extra Box! Posted by Brandy on March 11, 2017. Subscription Box Coupons & Deals / ONYXBOX Coupons Make the first comment! Hello Subscription may earn compensation via links in this post. Read the full disclosure Save for LaterGo to commentsNotification ONYXBOX has released a new coupon code – get a second box free when you subscribe with code EBB!