When you place your iPhone over an Apple Pay card reader, it will always pull up your default card. It may work well for you, but it may not be the card you want to use. The problem is that you do not have time to change cards before the payment goes through. It can cause you to use your unpaid debit card to buy groceries when you could have snagged 2% back with your Apple card.
That's why we recommend you open the Wallet app first. Here you can quickly change cards just by pressing them so you can use what is most meaningful to you. Balancing too high on the Apple card? Just switch to another debit or credit instead.
When selecting a card, simply place the iPhone over the card reader as usual and then follow the on-screen instructions to complete the payment with Touch ID or Face ID. If you switch to a reward card, such as one for Starbucks or AMC, you will probably scan the bar code instead.
There are actually some ways to open the Wallet app. You can always tap the app itself on your home page, but you may prefer to double-click the Home button or the Side button from the lock screen. The latter saves a lot more time and doesn't require you to dig through your apps every time you have to pay for something. Just know that it requires that you save a debit card in your wallet in advance, not just a reward card.