There are two ways to pay in a store using Apple Pay on your iPhone 8 Plus:
Either by resting your finger on the Home button / Touch ID sensor and holding the top of your iPhone close to the contactless reader (within 1 inch or so) until you see Done and a checkmark on the display.
Or by preparing your iPhone in advance, in which case you only need to hold it near the contactless reader to complete the payment.
Your description suggests that you are preparing your iPhone in advance:
- Open the Wallet app or, from the lock screen, double-click the Home button.
- If you wish to pay using a different card than your default card, then select your preferred payment card.
- Rest your finger on Touch ID (except in Japan).
- The display shows "Hold Near Reader".
- Within the next 60 seconds, hold the top of your iPhone near the contactless reader. Your finger does not need to be resting on the Touch ID sensor during this time.
- Done and a checkmark are displayed when the transaction has been completed.