Dear Kenaberdeen
In the Designer, there is a spec code (E90) that should do this for you.
If you open the Spec List, look under Invoice Header/Invoice Totals and you should see the E90 "Net invoice total + Current balance".
If you use Retainage, there are other fields that can be used as well further down on the same list.
After adding the field(s) to your invoice spec, remember to save the file before you close the specification again.
Hope this helps!
Regards,
Softrak Support