All of the fields you see in the invoice header can be referenced in our templates in this format:
<%=oInv.invtotal%>
oInv is the 'object', while .invtotal is the 'property' or in this case the fieldname.
Occassionally, the object could be called just 'invoice'. To see a list of valid 'properties' or fields, use the invoice report, then click the link to the view the invoice, then click the 'edit invoice header' and you will see the specific field names (aka properties).
An example of a formula you could use inside of the screenconfirmation.wc (the template commonly used for sending sales conversions to search engines is below.
<%=oInv.invtotal-(oInv.handling+oInv.tax)%>
This formula would return a value as 'text' that was the invoice total less the handling and tax, ie the order sub total.
