I want to welcome you to our forums. I can share some insights about the California DE9C form.
When you electronically file the DE9C form in QuickBooks, you don't need to attach the voucher manually. The system will automatically file the report including its attachments to the California e-Service site. You'll only need to attach the voucher if you are instructed by the agency to file the form by mail.
You may want to check out these articles and publications to know more about submitting the form: