Payflow Pro

Use Reference Transactions

This feature helps reduce your PCI compliance liability. Instead of storing card details in the WHMCS database the ID of the last transaction is used to make repeat charges. The limitation with remote card number storage is that changing payment gateway would require clients to re-enter their card details.

Contrary to what you might think, you should not tick the setting to "Disable Credit Card Storage" in Setup > General Settings > Security as that will disable entry by a client.