They will quote an hourly rate for their time and tell the customer that they work on a time and materials basis. That means X many dollars per hour of time, plus the cost of parts and supplies. Most shops also charge for research time, shipping of parts, recycling, rags and so forth.
You must be 18 years old to get a credit card by yourself. While you can legally get a credit card as young as age 18, the law requires you have your own steady source of income to qualify for a credit card. Unfortunately, a monthly allowance from your parents doesn't count. It's for your own good.
Most U.S. locations accept bank debit cards with the Visa or MasterCard logo as credit identification at the time of rental if you are at least 25 years of age. In most cases, U.S. locations will perform a credit check for debit card renters to determine credit worthiness at the time of rental.