There are some unfortunately unsurprising economics lessons to be seen in what is starting to result from interchange regulations. First, is that there when the government sets prices there are often unintended consequences:

…McDonald’s and other U.S. retailers that rely on a high volume of small dollar transactions could see an increase in their debit card processing costs, because prior debit costs for smaller purchases had lower fees….

…Richard Peck, 7-Eleven’s senior director for corporate finance, said the company’s gas stations and convenience stores will likely see a mixed impact from the capped fees… But for smaller, everyday purchases, executives said those costs will likely lead to price increases for consumers.

Oops, I don’t think that was supposed to happen. Another lesson is that regulation is often a slippery slope. Walmart seems to think that is the case anyway:

Michael Cook, treasurer for Wal-Mart, said the discounter viewed the debit fee overhaul as a precursor to overhauling credit card processing fees.