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.
Fortunately, the Credit CARD Act of 2009 raised the minimum age to apply for a credit card to 21, unless a parent or guardian cosigns on the application or the applicant proves that he or she earns a steady income. That said, there are plenty of benefits to establishing a credit history at a young age.