Easiest solution:
Group on product name
Create a Summary as a SUM of sales field at group level 1.
Place that summary on GH1 (it will default to the GF1 but this will work in a header).
Suppress details (and GF1 if you want).
Aother option to use what you already have with Running Totals.
Group on Product name
Adjust your RT to reset on group change
Place this RT on the group footer (running totals do not work in headers).
Place product name field on GF
Suppress the Header field and suppress the GH when section is blank
Suppress details row.