Make sure the amount you were approved for is greater than or equal to the amount being charged by the store (including tax), as this is the most common cause of cards being declined.
If you’d like to use the Zip app with Apple Pay, please tap on Shop in the app, place a purchase request, and then tap the "Pay with Apple Pay" button once your Zip virtual card has been created for your order.
You may need to have the store try entering the card information manually. Tap Shop and you should be able to read the full card number, expiration date, and security code.
Some stores may not accept the type of card used for the Zip app. If you are sure the amount of the purchase is less than the amount you were approved for, but the charge is still failing, you will need to use a different payment method for that transaction.