Downloading the Invoice to your system and sending it through your native email methods.
Sending it directly from Salesforce
You will find a Button (1 in the above screenshot), “Send Invoice PDF as Email” on Invoice detail page.
When you click on that, you will be redirected to the page where you can select one of the Contacts (related to Account of Invoice) and you can add other emails as well in CC and BCC. You can use Salesforce Email Templates also along with their attachments (if any).
We will be adding the Email Template Attachments along with the Invoice PDF attachment to the Email.
Video provides an overview of sending Invoice from Salesforce.
Downloading the Invoice and Emailing it
On the detail page of every Invoice Record in Salesforce, you will find a button (2 in the above screenshot), "Download Invoice as PDF".
This will produce the QuickBooks Online-generated PDF of the Invoice, you can save to your computer and then email. As this PDF is generated by QuickBooks Online, it looks exactly the same as if you had generated it within QuickBooks Online.
Save the PDF to your computer, and from there you can attach this Invoice PDF to an email.