You can add your card to Google Pay to make payments in stores or online wherever Google Pay is accepted. You can even add your card before it comes in the mail!
Here's how to link your card from the Step home screen:
Tap on into your account
Tap Manage Card
Tap on the icon for your device's digital pay system at the bottom of the screen
Follow your phone's instructions to add your Step Card to your digital wallet
Why Mobile Wallet and Google Pay are different
If you added your Step Card to your mobile wallet, you might expect it to be available as a payment method in the Google Play Store. However, Google differentiates between payments linked to the Wallet on your phone, and the payment methods linked to your Google account/Google Pay.
In order to use your Step Card for Play Store and online Google Pay purchases, or in-app purchases, you'll need to add the card to your Google payment methods. Check out this quick tutorial from Google for the full instructions.