NEW YORK (CNN/Money) – That Whopper or Big Mac may be costing you more than you think ... if you charge it.

In the first three months of this year, Americans charged $6.6 billion to pay for their burgers, fries and other fast food, according to CardWeb.com.

For the full year, CardWeb.com projects that consumers will charge more than $30 billion on fast food, or about 21 percent of annual sales at fast-service restaurants.

If you charge your value meal to a credit card on which you're carrying a balance, it may turn out to be a very expensive bargain, since the costs of not paying off at least some of your balance on time are increasing.

One of those costs come in the form of fees. Many issuers have been lowering the ceiling on balances for which the highest late fees and over-the-limit fees apply, according to CardWeb.com. For example, MBNA, the third largest issuer, will now charge a $39 late fee if your balance is over $250. In 2004, it only charged that amount on balances over $1,000.

Penalty rates also have been trending up. Consumer Action, a consumer advocacy group based in San Francisco, found penalty rates in the past year to be as high as 35 percent. It was just over a year ago that issuers' penalty rates were breaking through the 30-percent ceiling.

What's more, the instances in which those penalty rates apply remain broad. They may apply if you make a late payment, miss a payment entirely, or go over your credit limit.

They may even apply if your issuer has what's called a "universal default policy," which has become increasingly common.

Under such a policy, even if you have a perfect payment history with your credit card company, you could be hit with a sky-high penalty rate if your credit score goes down because of a chink in your payment history with another creditor.

The hit to your pocketbook won't be negligible. If you're carrying a $10,000 balance, a hike of 14 percentage points (say, from a 16 percent standard APR to a 30 percent variable default rate) would cost you over $1,000 a year extra in minimum payments alone.

That's why you should pay at least the minimum owed on time every month for all of your credit accounts and loans.

Credit card users who wish to pay off their balance in full and on time may have less time to do so. Grace periods – the number of days you have to pay off all new purchases without their being subject to interest -- have been getting shorter.

According to CardWeb.com, MBNA will now end its grace period by the payment due date, instead of the end of the billing cycle, which is usually a few days later.

Keep in mind, too, that some cards' grace periods will only exempt new purchases from interest if you are not carrying a balance on your purchases from prior months. So even if you have every intention of paying what you owe in full next month, everything you charge until then – including those fries – may be subject to interest.

Granted, not everyone can be a freeloader – in credit card parlance, that's short for cardholders who pay their balances in full and on time every month. And the card issuers would rather you not be, since customers who carry balances tend to be more profitable to them than those who don't.

According to an article in American Banker this month, card issuers took in $136.3 billion in revenue last year. Of that, 37 percent came from fees and the rest from interest. What's more, the fee income in 2004 rose 2.5 times faster than interest income.

So next time you charge that Happy Meal, you might consider just whom it is you're really making happy.

Jeanne Sahadi writes about personal finance for CNN/Money. For comments on this column or suggestions for future ones, please e-mail her at everydaymoney@cnnmoney.com.