Do you have these relationships?
If so, you can set up a summary report on a layout that is based on TimeSlips and you can group the records by Expenses using a sub summary report that is "sorted by" the name or ID of an expense record. Summary fields defined in TimeSlips can then compute the needed sub totals.
Thank you for your response!
I don't have that relationship defined. The related time slips and related expenses are really a part of each individual invoice. Ultimately I'm trying to print out individual invoices that reflect invoice specific info in the header, and time/expense data in the body, with totals at the bottom.
Then I recommend that you change your design so that you do have those tables and relationships. A portal to time slips will allow you to list a fully flexible number on any given invoice.