You have a summary field for total tax and and a summary field for total sales, correct?
Use this in a calculation field:
GetSummary(taxSummaryfield; breakfield ) + GetSummary(SalesSummary; Breakfield )
Use your field names and put the name of the "sorted by" field your sub summary part specifies in place of "break field". Now put this field into the sub summary part of your layout.
I know I have to get into Gets in depth, but hadn't cracked them yet.