Your first two attempts to perform the check are free, and 99% of people will complete the check successfully on their first attempt. If you don't pass on your first attempt you can do the check again, but first read the section on ID check failure - common causes. After two free attempts you will need to pay our regular fee of $5 to perform the check again.
You can start and stop the verification process and return to it as often as you like. However, once you complete the check and on your smartphone see the success page with a blue tick and words 'Registration completed successfully', it means you have used one attempt. Any attempts after the first two will require a payment of $5.