Here is the exact wording from the special offer details:
This offer is good for a $15 discount on one qualifying order placed. The qualifying order must be placed during the promotion period, which starts on the day the promotional email is sent and ends 6 days later (at 11:59 PM PST). Offer does not include shipping and handling charges or taxes associated with order. Customer must enter a reward code at the Shopping Cart page to receive the offer. Merchandise must total $15 or more before tax and other charges. The entire $15 reward must be used in a single transaction.
So if you were able to get the $15 credit, I'd recommend using it within the next six days. Otherwise, I'm pretty suspicious it will no longer be available for you.