Hi,
I have very recent experience in developing Google Sheet Addon and I have previous experience in working with FB API as well.
So let me tell you what I understood from the job description:
You need to send a message to your friends from a selected row. And one row may consist of message text, attachment, and columns of friends.
Send API - Messenger Platform has rate limits. Can you please explain, "User can send 10 free messages a day or unlimited for $24 per year (depending on if there are costs relating to the FB API)." ? Do you need users to pay for this service?
Kind regards,
Sazzad