I tried to answer this in your other post and am not sure where you were having problems.
Greoup on Invoice_no
Create a formula called 'flag' as:
If {table.Stock_num}=" your certain product here" then 1
Now insert a Summary using this formula field 'flag'
Click on the Sigma sign (looks like an E)
select the formula field name 'flag'
calculate the summary as: SUM
Summary location = group footer - select the invoice number group
This wil insert a summary field in the group footer.
Go into your select expert cahneg it to a ghroup Select and use your condition here
SUM(flag,Invoice_no)>0