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.
The legal age in the United States to obtain your own credit card is 18 years. Fortunately, there are other options available to you. First, you can obtain a prepaid debit card which you charge up by sending money to the card issuer, and then use that card as you would a regular card, everywhere Visa or Mastercard is accepted.