Automatically selecting the attachments to the email box when creating an invoice isn't currently an option. You’ll have to click the Attachments feature from the invoice page. Then, manually choose the documents on the uploaded attachments. Let me guide you on how.
Go to the Sales menu.
At the bottom left of the page, tick the Attachments.
Browse and select the document you want.
Choose Open, then Save and close.
The Sales menu lets you view the status of your sales transactions. You can even create and edit transactions from there. To learn more about this menu, you can check out this article: View sales transactions.
Furthermore, you can also visit our blog post to know the latest product and feature updates: QuickBooks Online blog.
Feel free to leave a comment on this thread if you have any other questions or concerns. Take care.