How do i let them enter their credit card the client

Welcome to the Community, @samwolfer. Thanks for sharing your first concern.

To let your customer enter their credit card details when paying their invoices, you'll want to connect your QuickBooks Payments account to QBO if you already have an existing account. 

Here's how: 

1. Sign in to QuickBooks Online as an admin.
2. Go to Settings ⚙ and then Account and settings.
3. Select the Products and services tab, then select Connect or Link Merchant Service.
4. In the window, select the QuickBooks Payment account you want to connect. Then select Connect.
5. Review the account info and make sure it's accurate. If everything looks good, select Connect.

Once done, sign out and sign back into QuickBooks Online. 

If you don't have an account yet, I'd suggest you sign up for QuickBooks Payments directly from QuickBooks Online.

For future reference, sign in to the Merchant Service Center to see your transaction history or processing fees service fees, and manage your settings.

Let me know if you have other questions. I'm always around to help. 

@samwolfer 

Afaik, QB Payment doesn't offer such feature. You need to have a 3rd party billing app with a portal to integrate with your QBO. Your clients will be able to track their bills or self update their payment details.