Blockcypher is used to check confidence levels of incoming BTC cash-out transactions that are below your 1-confirmation threshold.
The details of 0-confirmation transactions are submitted to Blockcypher's API which checks for reasons why the customer's deposit may not get confirmed, such as spending unconfirmed change, unreasonably low fees, and potential double spends.
The threshold is a value 1 to 100, which is customizable. If the incoming transaction is below this threshold, the machine will direct the user to the 1-confirmation flow which requires their phone number and to await the first confirmation before redeeming.
Blockcypher's services are free for 15 confidence checks per hour. Beyond that, you may need to upgrade to a paid plan. Their portal will indicate how much your API token is being used.
To create an account, visit: https://accounts.blockcypher.com/signup
After confirming your email address, the main page of the Blockcypher portal will show your API token.
Input this and a confidence threshold into the admin under 'Third Party Services / Blockcypher'. 50 would be a reasonable value to start with.
Then, within the 'Global Settings / Wallet Settings Panel', choose Blockcypher for the Zero Conf value, and click Submit.
Test by placing a 0-confirmation transaction at your machine.