This endpoint allows you to send a finalized, imported customer invoice or credit note
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 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.

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Warning: If the invoice is eligible for electronic invoicing, sending

it by email prevents it from later being transmitted through an approved
platform (PA – Plateforme Agréée). For invoices within the scope of the
e-invoicing reform, transmit them through the approved platform instead of
sending them by email.

The subject and body come from the company's default billing email template
for the customer's billing language, unless email_template_id is set.
An invoice collected by GoCardless uses the GoCardless billing email template
and does not accept email_template_id.

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This endpoint requires the following scope: customer_invoices:all