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.
You must be at least 18 years old. Because you need to take a credit check before you sign up, you need to be at least 18 years old to get a pay monthly or SIM only contract. If you're not 18 yet, don't despair. You can either ask an adult to take out a pay monthly contract for you (the credit check and bills will need to be in their name), or check out our great range of pay as you go phone or broadband deals.