For mail in payments, you may not want to give them some information such as links to access certain materials, username, passwords, etc. until you are sure they have paid.
As a result there is a need for two types of thank you emails:
i) For people who have paid (this will contain all event details)
ii) For people who have not yet paid (this will omit some details and instead provide them things like the address to mail their cheque to)
Also with respect to pay later customers. Sometimes someone will sign up for a pay later option (such as mailing us a cheque). Once we receive the cheque we will change the status to paid. This should trigger the email for people who have paid. But on rare occassion we later discover one of the cheques have bounced and will have to change the status from paid to unpaid.
Right now with DT Register, there is no email that can be sent for someone who has paid. The only support that exists is for a change in paid status regardless of direction (i.e. Paid -> Unpaid vs Unpaid --> Paid). Depending on which direction you are going, the emails should obviously be very different.
But the solution I proposed (i.e. having a initial message for those who have paid and another for those who have not paid; as well as an email for someone changing status from unpaid to paid should solve all identified issues.