Credit cards basics

I’ve seen a lot of advertisements for companies that consolidate credit card debt. Are these legitimate?

Can I make issuers stop sending me credit card offers in the mail?

I am receiving many offers for new credit cards since I moved into my new home or took out my mortgage. Can I make the offers stop?

Why wasn't my online payment credited to my account on the same day I made it?

Can my card issuer change my credit card from a MasterCard to a Visa - or from a gas card/department store card to a MasterCard?

How long can the card issuer take to resolve my billing error or dispute?

Can they charge me interest on a charge I told them I did not make?

Can my credit card company charge a fee based on how I paid my bill, such as for making a payment over the phone?

When do I need someone to guarantee or co-sign, or be a joint applicant on my credit card account?

When does a credit card company decide what interest rate to offer me on a credit card?