When you have cost but do not hold inventory like in your situation, open the chart of accounts and click new, select COGS and for detail type select Cost of Sales (COS). Then make the COGS account inactive.
Post all purchases for the order to COS
screws, glue, sandpaper, etc are an expense, use shop supplies expense when you purchase
Unless you are taxed as a corporation, your time/labor has no cost in business accounting.