How do I add a payment button so a customer can pay for an invoice with a credit card.

You'll want to set up QuickBooks Payments, mmhomeimprovements.

1. Click the Gear icon.
2. Select Account and Settings.
3. Select Payments. In the QuickBooks Payment section, select Learn more.

Check out this article for the detailed steps: Sign up for QuickBooks Payments.

If you have an existing payments account, connect your QuickBooks Payments account to QuickBooks Online.

Here's an article for future reference: Find out when QuickBooks Payments deposits customer payments.

Stay in touch with me if you have additional questions.

@mmhomeimprovements 

As additional option. It supports QBO

https://melio.grsm.io/quickbooks