Automatic send out invoice every month

Can anyone point me in the right direction for automating invoices going out every month?
At the moment we have to manually go into each Order and press the SEND INVOICE button.

I’ve tried creating an Campaign with an email in it but I cant seem to attach the invoice or merge the Invoice fields.

Thanks Lisa

Maybe a bit more context? By description it sounds like a subscription but I’m sure there are other factors?

Hi, No we don’t sell subscriptions. Our sales are done in person and we then raise an order in Keap.
Thanks Lisa