Few small retailers understand the rates charged by credit- and debit-card processing firms  and many suspect they're getting ripped off, according to a new survey.

Of 400 retailers surveyed nationwide, only 21 percent said they understood the often complex fee structures, while only 26 percent felt they were being treated fairly, according to Heartland Payment Systems, a Princeton, N.J.-based payment-processing firm.

Business owners may have reason to worry. Last week, the attorney general of Missouri sued First Capital Bankcard, a Kansas City, Mo., payment-processing firm, for deceptive practices, including charging inflated fees.  More than 35 merchants had complained about the firm, with reported losses as high as $2,000, according to the attorney general's office.

"Running a small business is challenging enough without having to worry about someone scamming you," Missouri Attorney General Jay Nixon said in statement.

A month earlier, the Federal District Court in Oregon froze the assets of Merchant Processing, a Beaverton, Ore.-based payment processing firm with thousands of small-business clients across the nation. According to the Federal Trade Commission, the firm allegedly lured merchants with promises of lower processing fees, instead charging additional fees hidden in contract fine print.

The trade commission is currently seeking injunctions against the company, along with refunds for its clients.

Credit- and debit-card payment processing  is one of the three biggest expenses faced by small and midsize retailers, according to Heartland Payment Systems CEO Robert Carr.

While Visa, MasterCard, and other credit card companies typically charge retailers about 2 percent per transaction, additional fees charged by payment processing firms -- which often also sell or lease the necessary hardware for merchants to accept credit and debit payments -- are generally lower, but can range widely from business to business and transaction to transaction.

In turn, these fees are absorbed by retailers in either higher prices or lower profit margins.

According to Carr, small-businesses owners need to know how these fees are calculated to better manage costs and protect themselves from fraud.

Last fall, he launched the Merchants Bill of Rights, a website devoted to improving transparency and fairness in the payment processing industry. The site, which is endorsed by hundreds of small merchants, trade groups, and other associations nationwide, identifies 10 fundamental rights for business owners. Among these are the "right to know the fee for every transaction -- and who's charging it," as well as the right to know "all transaction middlemen."