Thanks for the reply Ewa.
So there is no way to simply trigger the invoice to send, in the same way it does when you click “Send by Email” on the website?
I simply want to send it, to the email for the contact that is already stored in FreeAgent…
My worry is that I need one API call to create the invoice, another API call to fetch the contact details, then another API call to “send the invoice”.
We’re planning on using FreeAgent to handle all our billing for hundreds of invoices that happen at specific dates in the month - I’m worried that if my approach mentioned is correct, we will hit API rate limits.
EDIT: I’ve just seen this post - which says you can post empty options through, however that doesn’t work for me, I have to fill in all fields. Including the message body which is far from ideal 
Many thanks,