You can't add fields or do calculations on your standard Sage 50 invoice but you can create a custom invoice using Intelligence Reporting to do this.

What is Intelligence reporting? I assume this is an upgrade of sorts?

It's a report writing tool that's included in Sage if you are on a Business Care plan.

I have installed the intelligence reporting. Now how do I add parts and labor totals to my invoice?

I have intelligence Reporting and I do not see how you can customize an invoice. I only see reports.

I  manually subtotal the items and put the total in the description area.  I know it would be nice to have a subtotal, but that is not going to happen in Sage 50 unless you consolidate time or expense tickets.