How can a customer store their credit card details?

There are a few ways.

Firstly it depends on whether you want to store the details so you can autodebit them, or whether they want to store them so they can make payments more easily

To store them for autodebit (pre-approval) find the customer in the portal and you will see the options for pre-approvals. You can launch the pre-approvals page to fill it out for them, or send them an email by pressing the email button.

Our customer portal also allows customers to store their credit card details but it doesn't allow for pre-approvals and is so that customers can pay invoices either one off or in bulk without having to re-enter their payment details.