This is available in current website version.
For invoices:
- Open Invoices list window,
- Select a desired invoice from the list
- Click the 'Email' icon on top toolbar,
- [A new window should open up] Select the 'Include the invoice in the body of the email'.
- Fill in other details appropriately, and click 'Send' button.
==========================================================================
For quotes:
I don't think the feature is available for quotes, but it is possible to send the payment link in the body of the email.
- Follow the similar steps as above from quotes list window.
- Put the payment link in the text-box that says 'You may enter a comment to be included in the email.'