I run a mobile diesel business and need to know where parts go.

As I said in the subject I run a mobile diesel business working primarily on farm equipment such as tractors and pick ups. When I make a bill I charge for labor and parts, I do not mark up parts so I am making no profit on the parts, often times they are bought at local parts stores such as advance with not professional account.

Do I count the payment for the parts as income and then deduct them or do I put them under the inventory/cost of goods sold or do I just not count the parts payments in my income? (the parts are paid for by me and then reimbursed to me in the bill to the customer).

Thanks