What if we want to use an invoice payment?

You can actually attach a file in the Autoresponder Email. The Autoresponder Email is the confirmation email that your client will receive after completing the form. Please check these guides:

Setting-up-an-Autoresponder-Email
How-to-Attach-a-File-to-Autoresponder-Emails

You may also enable the PDF attachment in the Autoresponder settings. In this way, your client will receive a PDF copy of the submission.

If you want to customize the PDF, you can use the PDF Designer. Here's a guide: How-to-Customize-PDF-Submissions-Report.

Hope that helps. Thank you.