This endpoint allows you to send a finalized invoice or an estimate by email to your customer. This requires that the PDF file for that document has been generated (this process can take a few minutes), so if you just created the invoice / estimate in our system, we may return a 409 error. You should retry the request in a few minutes - if you receive a 204 response, that means that the email is on its way. For more information about email sending, please read this guide.