Paying by credit card may be easy and convenient, but now, it also could cost you more.

Terms of a multi-billion dollar settlement between credit card companies and merchants went into effect Sunday. Merchants can now charge you between 1.5% and 4% extra for using your credit card to make a purchase. Merchants pay a processing fee to the credit card companies for every transaction, and now, they have the right to pass that cost on to their customers.

Charging you more for using plastic isn't mandatory, and it isn't clear how many stores will do it. Those that do will have to post that information at the register, and on their website.

There are three key things for you to know as a consumer:

• This applies to Visa and Mastercard only. American Express isn't part of the settlement; it doesn't allow retailers to add surcharges.
• The surcharge applies only to credit cards. If you're paying with a debit card, you're in the clear.
• It won't apply nationwide. Ten states have laws against credit card surcharges. So, if you're using a credit card in California, Colorado, Connecticut, Florida, Kansas, Maine, Massachusetts, New York, Oklahoma or Texas, charge away.

The lesson here is clear, says HLN Money Expert Clark Howard: pay with cash. You'll avoid the credit card surcharge, and you'll spend more carefully.