• Tap the Invoices tab.
  
• Click the Unpaid or "Overdue" status from the drop-down list.
  
• Tap/Select the invoice : Lands to the Invoice Details page.
  
• Tap the action button from the Android device.
  
• Tap the option Send Reminder.
  

Ensure that the To field is filled with your customer's email address. Check the mail content and tap the arrow/send option.