I believe you can get it from both sources (retailer and developers), but you will have to get in touch with HP Support for that.
HP Support: http://www8.hp.com/us/en/contact-hp/contact.html
Additionally, you should get in touch with the retailer. They will provide additional information. Do not try to enter the coupon any more, because if it gets activated after all, you won't have the possibility of asking for a refund anymore.