Were you able to find out how to have the 2 emails on monthly invoices?  I am running into the same thing where I can save it to the customer information (our amounts are different monthly also so I cannot set up a reocurring monthly invoice) but in order for it to appear on the next emailed invoice when I got to send it, I have to remember to manually type (or copy and paste) the second email each month.

Thanks, Leslie