What we are trying to accomplish is to pull variables from the Invoice into the Standard email which is sent out by the system.  For example, the Invoice has field variables which are keyed by the user (such as a custom field called PO number) or several system calculated variables (such as Sub Total, Total, Invoice Due Date, or Balance Due). 

If you look at the Standard Email's subject line- the system is pulling in the variable for [Invoice no.]. We would also like to include other Invoice variables such as [PO Number] in the Standard Email's subject line. Why is this not possible?

This seems like elementary functionality which is missing and which opens up room for error as the user has to manually type in these variables each time an invoice email is sent.

For Example, we would like the email to be able to pull in the following variables:

Subject:

Invoice [Invoice no.] due [Invoice Due Date] from [Company Name] for PO [PO Number]

Message to the customer:

Please find your invoice, as attached, in the amount of [Invoice Total]. Kindly remit payment prior to [Invoice Due Date].