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.
To be eligible to apply for their own Medicare card, young people need to be at least 15 years old. A young person would then need to complete a Medicare Copy/Transfer Application form. These are available at Medicare Offices or you can phone toll free: 132 011.