They're baaaack! Swipe fees, those charges Visa Inc (NYSE:V) and Mastercard Inc (NYSE:MA) impose on merchants for the privilege of allowing customers to use credit cards in their stores, are back with a vengeance and threatening to raise the cost of your purchases by as much as 4%.

As part of the $7.3 billion antitrust settlement with the financial services industry last July, the credit card issuers and a number of large banks including Bank of America Corp (NYSE:BAC) and Wells Fargo & Comapany (NYSE:WFC) agreed to reduce the amount they charged merchants for processing credit card transactions, but only for eight months, after which they'd go back up. In addition stores that previously were prohibited by Visa and MasterCard from passing along the cost of processing the transaction to consumers would now be permitted to do so. That deadline expired Sunday.

It's easy to understand why the financial services industry was eager to settle the suit. Swipe fees are big business. Business Insider estimated credit card companies generated $50 billion annually in interchange fees, as they're called, so paying a $7 billion fine to settle the case and lowering the rate for eight months -- which cost them an estimated $1.2 billion -- is a good deal to essentially keep the status quo. Moreover, merchants are now very limited in their ability to bring suit against the credit card companies in the future.

Swiping from your wallet Of course, merchants have an option to eat the cost of the transaction rather than pass it along to the consumer, and in an economy that remains weak many are hesitant about increasing costs that might drive customers away. Wal-Mart Stores, Inc. (NYSE:WMT), which opposed the agreement, said it would "cost consumers tens of billions of dollars each year," while Target Corporation (NYSE:TGT), another vocal opponent, called it "bad for both retailers and consumers." It said it didn't plan on imposing the surcharge on its customers merely to "allow Visa and MasterCard to continue charging unfair fees."

The theory behind the swipe fee is that it is paying the credit card company for the risk inherent in the transaction as well as the cost of the transaction itself. You've made a purchase and received merchandise and the store has gotten paid right away, but the credit card company typically has to wait at least 30 days to get its money when you pay your bill. Of course, the credit card companies are also charging you interest if you carry a balance on your bill, but they say that's not enough to cover the costs. The Merchant's Payment Coalition might beg to disagree. They say only 13% of the swipe fee goes to processing the transaction; much of the rest goes toward its marketing its cards to other consumers.

American Express Company (NYSE:AXP) and Discover Financial Services (NYSE:DFS) actually allowed merchants to charge swipe fees so they weren't party to the imbroglio. But they told retailers they had to treat all credit card companies the same, so if they imposed a fee on an AmEx card, they'd also have to impose one on Visa and MasterCard purchases. And since they were effectively prohibited from doing so by those issuers, the merchants imposed no fees at all.

But it's also one of the reasons why AmEx cards are accepted at fewer places than Visa and MasterCard. Since they charge higher transaction costs, it costs the merchant more to process a transaction using them.