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.
People with bad credit have the fewest options for credit cards. That's because few credit card issuers want to take the risk of loaning a credit card they might not get paid for. The lower your credit score, the harder it will be to get approved, but it's not completely hopeless. Though it may be hard, you can get a credit card with bad credit.