In order to void the invoices on the iPad, you need to proceed with the following steps:

1. Go to the "Billing" tab.
2. Select the invoice and click on "View Details".

3. Click on "Void" at the top.

4. It gives a pop up asking you to confirm the voiding process.

5. Then, hit "Save" at the top right corner.

6. The invoice gets voided.