I'd be glad to walk you through the steps adding the Pay Now button on your invoice. You'll have to sign up for QuickBooks Payments.
Please note that the application will take 2-3 business days. You'll be notified via email once it's approved.
Now, you can start sending invoices with the Pay Now button and your customer can easily pay them directly online.
You can learn additional information on signing up for QuickBooks Payments in this article.
Please tag me directly in the comment below if you need anything else.