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.
1.5 Minimum age and wage for credit cards. In the UK, you need to be 18 or over to sign any credit agreement. If you are under the age of 18 and you sign a contract, it will not be binding. To apply for a credit card, you will need to sign a credit agreement and therefore must be at least 18 years of age. All credit card companies set out their own acceptance conditions that must be met by any applicant before they are awarded a credit facility and a credit card to access that facility.