Pre-Paid Invoices not being emailed

We have credit customers and pre-paid. Here is the scenario, we raise an order and the customer pays so there is zero balance. Once the order gets dispatched it gets converted to an invoice (which will then show fully paid and closed) The following day when I email all the previsous days invoices - this one is not on there.

I am assuming it's because as soon as it was converted to an invoice it would have been "closed". I also I did a test with the same customer (converting to an inv etc but leaving out the payment) and voila it was on the email list. So it is definately something to do with the payment.

How can I get MYOB to still add unsent paid invoices this to the email list? 

Many thanks