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.
So if you are over 18 you can get one in your name, but try to look around for one that doesn't need a cosigner or deposit. If you are under 18, (but over 16), you can get one with your parents, but they are like, the main signers, just with your name on the card.