So I thinkt he easiest solution is to not filter your data but rather zero out records that do not meet your conditions.
basically move your select criteria to a formula field
if {TRNSTK.CUSTOMER} startswith {?Customer} and
{TRNSTK.PERIOD} = {?month} and
{TRNSTK.YEAR} = {?Year}and
then {TRNSTK.Units} else 0
//guessed at the units name
now you can insert a SUM of this formula field at the grouplevel 1 to get total units per Brand
If you want 25 that show to be dynamic based on volume you can use TOP N group options.
Other wise you will have to write a formula for your 20-25 that you want to show and groupng the rest under 'other'. This would replace your field to group on for group 1 (Brand)
Edited by DBlank - 19 Jan 2012 at 6:30am