The offer is valid through December 22, and the promo code for $20 will be delivered within two weeks of your purchase. Apple says that you will then have until January 31, 2019 to redeem the code and afterwards it will expire.
Apple has been increasing its holiday promos for Apple Pay recently, running discounts at numerous stores in December, including Pier 1, Groupon, JanSport, Oakley, Rakuten, and more. These websites and apps will keep up their Apple Pay offers for one more day, expiring on December 19.
Prior to the holiday deals, Apple this year has partnered with companies like Ray-Ban, Postmates, Under Armour, and Panera Bread for its Apple Pay promos. Previous sales have also been themed around major holidays and events, including summer vacations, back to school, Thanksgiving, and now Christmas.