It's simple really. Though I do admit it can be difficult to "wrap your head around" in the process of understanding it.
You treat your parts and other things that you actually sell to customers, as inventory. It doesn't matter if you sell it as is, like in a box "off the shelf" directly to the customer, or if you sell as as "a part of" the repair service you provide.
You are already including what you paid for the part on the invoice you issue to the customer. So the customer is paying for that part, and the costs billed that exceed what *you* paid for the part is your labor. So you have no need to report what you paid for that part anywhere in your tax return outside of the "Inventory/Cost of Goods Sold" section.
Since you don't actually "carry" an inventory that you sell to customers, that makes COGS easy for you.
BOY (Beginning of Year) Inventory balance $0 (What "YOU" paid for the inventory in your physical possession on Jan 1 of the tax year)
EOY (End of Year) Inventory balance - $0 (What *YOU* paid for the inventory in your physical possession on Dec 31 of the tax year)
Cost of Goods Sold (COGS) - $5000 (What *YOU* paid for the inventory you *actually* *sold* in the tax year. This is the amount that will be deducted from your gross taxable business income so you aren't taxed on it.)
It's that simple. But understand this.
Your BOY Inventory balance *MUST* match your prior year's EOY Inventory balance. (In your case, zero) If it does not match, then you have some 'splainin' to do to the IRS and they flat out will *not* accept any reason or excuse you may give them, other than "I screwed up. How much is my fine?"
