Can this out-of-inventory order be called a business expense?
Usually we buy plenty of doors at the beginning of the year and then charge the customer for parts and labor a the time the sale is made. We recently had a customer that didn't want a door from our inventory, so the customer prepaid for a door that had to be ordered. Is the door still a business expense to us even if we had to pay for it from our business bank account although we got a check from the customer?
It would be a reimbursable business expense. You paid for the door, so it's a cost-of-goods-sold expense. The amount you charge the customer is income to you. If it's dollar for dollar, then you can run it as income or just reimbursed business expense.