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.
You have to be 18 although even when 18 you could be decilned as it has something to do with your income as well. I recommended getting a credit card when your around 20 as you will have more knowledge of money and not just max it out also at the age of 20 you will still have a good credit report.