Welcome to the Community, ProControlMan.
 

When a client goes to pay their e-invoice, they'll see a option titled "Save payment method for next time (sign in or create an account to save)".

Clients can tick this box while signed in to their account, then click the Pay button. This way, their payment details will be pre-populated for future online payments.

I've additionally included a detailed resource about using QuickBooks Payments that may come in handy moving forward: Take & process payments
 

I'll be here to help if there's any questions. Have an awesome Friday!