Cost of Goods Sold (NO Inventory) VS Supplies as Expenses

[Champ77]

Trying to figure out the correct way to classify parts for an auto repair shop. If I order parts or tires for the vehicle that we are working on, will whatever I paid for those parts be categorized under COGS or parts as an expense?

All those parts are gonna be in the invoice that the customer pays. 

Where should I put the cost of the parts that I buy from the vendors?