I'm a carpenter &buy materials for the job when it begins. I charge customers for materials. Would that go under materials & supplies for expenses or under cost of goods?

This is an example of what I do. I did a job for a deck and I charged the customer about $800 for supplies. The cost came up over $800 so I paid for it out of pocket, but then made sure to add it to the invoice after. I also charged for my time so it ended up coming to a total of $2100. How would I put the supplies in? Would they go under expenses for materials and supplies or inventory? I usually never have anything left over from a job. The materials are always bought when the job starts. This is my first year so I'm a bit confused.