QuickBooks Self-Employed doesn't support attaching files, such as a W-9, directly to an invoice before sending it out, Garzatron.
As a workaround, you can export or save your invoice as a PDF and then manually send it with your W-9 form in a single email outside of the QuickBooks Self-Employed.
Here's how to export:
- Go to the Invoices tab.
- Click the drop-down arrow under the Action column.
- Select Export as PDF.

Feel free to revisit this conversation if you have additional questions.