It looks like Google is doing their best to make it even easier to use Android Pay effectively and take advantage of any discounts available on the service. They created a new promo page for Android Pay that lists out the current deals which can be found when you use it.
Some apps will automatically apply the discounts when you choose the mobile wallet service at checkout, while others require you to type in the ANDROIDPAY code. Here's a link to the landing page to check out the Android Pay current offers: Android Pay Promotions.