Completing the below steps will enable and display Afterpay as a Payment Method.
Navigate to Shopify Admin.
Navigate to Settings > Payment Providers
Under the ‘Alternative payments’ section, click on the ‘Select additional payment method’ dropdown.
Select ‘Afterpay’ from the dropdown options.
Enter the Merchant ID provided by Afterpay into the Merchant ID field.
Enter the Secret Key provided by Afterpay into the Secret Key field.
Be sure not to check the ‘Enable test mode’ box, as pictured below. The integration will not work if this box is checked.
Click the ‘Activate Afterpay’ button.