Tap the Menu icon from the bottom menu bar
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Select Invoices 
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Select the invoice you'd like to pay by tapping it. Note that you can pay an invoice in advance of it's due date if you desire.
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On iOS, tap Choose Payment Method. On Android, Tap the payment icon 

Tap the payment method you'd like to use. If you haven't added payment information to your account yet, you can add it during this step.
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Tap Confirm Payment