So many people fail to understand that the cost of goods is nominal compared to all of the other expenses a business has. There is rent &/or loan payments, phone, heat, lights, taxes, INSURANCE, labor, advertising, accounting, legal fees, equipment initial cost plus maintenance thereafter, employer's share of Social Security, etc. etc.
As a bank commercial loan officer for 40 years I had hundreds of people wanting to get in business for themselves thinking that their only real expense of doing business was the cost of the goods they plan to sell. Besides all of the above expenses that road side service had the additional expense of a truck with all of its equipment plus the driver that does not generate other income 24/7.
Your mechanic friend did you a real favor I sure hope that you thanked him.
Dave