In WHMCS, there isn't a direct setting for "Attempt Only Once" for payment processing. However, you can achieve similar behavior by configuring your payment gateway settings and adjusting your invoice handling procedures.

Here are the steps you can take:

1. Log into WHMCS: Access your WHMCS admin panel using your username and password.

2. Navigate to Configuration: Click on the "Configuration" tab in the top navigation menu.

3. Payment Gateways: Under the "System Settings" section, click on "Payment Gateways."

4. Select Payment Gateway: Choose the payment gateway for which you want to configure the behavior.

5. Configure Retry Settings (if applicable): Some payment gateways provide settings for configuring the number of retries in case a payment fails. If your gateway allows this, you can set it to "1" to attempt the payment only once.

6. Adjust Invoice Handling:

   • If a payment fails, consider setting the invoice status to "Unpaid" or "Pending" (if it's not already) and notify the client to manually make the payment.

   • You can also set up automated reminder emails to notify clients about unpaid invoices. This can be configured in WHMCS under "Automation Settings."

   • Consider configuring late fee settings to encourage prompt payment.

7. Monitor Payment Logs: Keep an eye on payment logs within WHMCS to track the status of payment attempts. This will help you identify any issues or cases where payments may need manual intervention.

8. Regularly Review and Follow Up: Regularly review the status of invoices and follow up with clients who have outstanding payments.

Remember, while it's important to automate payment processes, it's equally important to have manual procedures in place to handle exceptional cases where payments may fail. Clear communication with your clients about your payment policies is also crucial.