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.
One of the changes had to do with the minimum age of card applicants. Here's how old you need to be in order to open your own account: Applicants who are younger than 21 years of age will need to have a co-signer (who is older than 21) when opening a credit card account.