You can add your card to Apple Pay to make payments in stores or online wherever Apple 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 into your account
Tap My Account (below your balance)
Select Manage Card
Tap Add to Wallet at the bottom of the screen
Follow your phone's instructions to add your Step Card to your digital wallet
Apple Pay is different than your Apple ID Payment methods
If you added your Step Card to your mobile wallet, you might expect it to be available as a payment method in the App Store. However, Apple differentiates between payments linked to the Wallet on your phone, and the payment methods linked to your Apple ID.
In order to use your Step Card for App Store purchases, or in-app purchases on your iPhone, you'll need to add the card to your Apple ID payment methods. Check out this quick tutorial from Apple for the full instructions.