Add values in the sub-summary part of a report layout?
I have two values in the sub-summary part of a report layout used for invoices.
The first value is a summary total of the value of all the "sales lines" of the invoice (this information is taken from the transactions table where invoice number is the same for each transaction), the second value is an overall value for shipping and is taken from the invoices table which also stores customer name, address etc. The two tables are linked by the invoice number.
For instance, an invoice has 4 lines of information for different products purchased. The layout repeats that section with data from the transactions table until all four lines are included. In the sub-summary, it then adds the values together and gives a total value of the goods sold.
Also on the sub-summary section is the single value taken from the invoices table for the amount charged for carriage. Because of the way the data is drawn from external CSV files, I can't push the carriage value into the transactions file as another transaction; if I could, it would obviously simply list it as another line and be included in the total. Also, many companies want a total of the goods shown separately, followed by any carriage charges incurred and finally an overall invoice total.
Should be simple, but how do I add those two values together in the sub-summary section to create the total value of the invoice?!