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.
If you are over 13, you can get a prepaid credit card, like a much Music card in canada, I beleive you can get them from most major credit card companies and banks as well though. You have to be over 16 to get it without your parents but you just have to be 13 to get one with your parents.