This article shows exactly what you need to do to pay an invoice on the mobile device using Clover. Tapping on the Pay button from the invoice will not work. Instead, you need to tap Preview. These screen shots show the process.
STEP 1: Tap on Invoice List from the app's main menu.
STEP 2: Tap on the invoice you want to pay.
STEP 3: Tap on the down arrow beneath the Action Bar. Do not tap on the Pay button.
STEP 4: Tap on Preview.
STEP 5: The invoice displays. Tap on Pay Invoice.
STEP 6: Insert the credit card information to pay and tap the orange button. If you are taking a partial payment or a deposit, you can change the amount to be paid. Then tap on the orange Pay button.