That is part of your record keeping and not normally added to your tax forms.  Start with total cost of inventory after expenses.  You could make that a miscellaneous expense on your taxes, but it is not how businesses keep their books and could lead to confusion.

are these customer returns and allowances or other returns and allowances?

customer items go on line 2 of schedule c

if these relate to cost of goods sold then net against purchases - there is no specific line for returns and allowances regarding purchases

if you talking about expenses reported elsewhere such as supplies (line 22) again just net.