Only 14.3% of customers at big banks had overdraft coverage in 2011 and 2012, according to the Consumer Financial Protection Bureau.

But for the minority of consumers who do opt in, overdraft fees can be enormously expensive. Around 8% of consumers paid overdraft fees at least 10 times per year, which cost them an average of $380 annually, the CFPB says.

Now a coalition of consumer advocates is calling for new restrictions on banks' ability to market overdraft coverage to their customers. After sending so-called mystery shoppers into branches in four states, the consumer groups say that branch employees often imply in their conversations with potential customers that they don't have a choice with regard to overdraft coverage.

"Even if the bank is following the rules in term of their written materials, what they're saying to consumers verbally is very confusing and sometimes downright inaccurate," said Dory Rand, the president of the Woodstock Institute, which is one of the groups that conducted the research.