But the report found that some businesses such as supermarkets benefited little or, in some cases, are paying more.
All told, the report estimated that the overall savings supported 37,501 new jobs.
But many business groups argue that the savings could have been larger if the Federal Reserve had set the cap at its original proposal of no more than 12 cents per transaction rather than 21 cents instead.
That would have ramped up the savings to $12.5 billion and more than 55,000 jobs, the study said.
But credit unions disagreed with the analysis.
Carrie Hunt, senior vice president of government affairs and general counsel for the National Association of Federal Credit Unions, questioned the report's findings.
"After two years with the Durbin amendment, retailers are reaping $8 billion in rewards while there is still no proof of any savings being passed along to consumers," Hunt said.
"In fact, consumer prices have continued to rise despite claims to the contrary."
Credit unions have seen a decline in their per-transaction interchange fees and have reported that in many cases that the fees are not sufficient to cover the costs of processing additional transactions, Hunt said.
They are considering a variety of measures to recover these losses, including eliminating or reducing debit rewards programs (27.6 percent) and eliminating free checking accounts (25.8 percent), among others.
"Ultimately, consumers will be hit with a double whammy if credit unions stop offering many of their low-cost, low-fee services.”
The Electronic Payments Coalition called the study flawed and argued that consumers aren't benefiting.
“As part of their lobbying tactics, giant retailers promised to lower prices for their customers if Congress passed the Durbin amendment," said Sam Fabens, spokesman for the Electronic Payments Coalition.
"Two years after implementation, retailers have taken home an $8 billion annual windfall while their customers still aren’t seeing a discount for using debit."