NEW YORK (MainStreet) — Small debit card purchases lead to overdraft fees, according to a new report from the Consumer Financial Protection Bureau.

The study said the majority of purchases hit with a median overdraft fee of $34 are under $24. That $5 latte you bought this morning could end up costing you $39 if you're not careful.

Overdraft fees are charged when there is insufficient money in your checking account to complete a transaction.

The study also said the majority of overdrafts are replenished within three days and if a consumer borrowed $24 for three days and paid the $34 overdraft fee, this would amount to a loan with 17,000% annual percentage rate.

"Despite recent regulatory and industry changes, overdrafts continue to impose heavy costs on consumers who have low account balances and no cushion for error," said CFPB Director Richard Cordray. "Overdraft fees should not be 'gotchas' when people use their debit cards."

The regulation, which took effect in 2010, called for consumers to opt-in for the bank's standard overdraft protection, which approves debit card purchases, even if there isn't enough money to complete the transaction, but still charges the overdraft fee. Should consumers not opt-in, or simply do nothing, such purchases will be declined without the fee.

There are few benefits to opting into such protection. You'll be slammed with fees when you least expect it. No matter how closely you monitor your checking account, even the savviest consumers can still fall prey to overdraft fees. Some consumers opt-in to avoid the embarrassing stigma of having your debit card purchase declined at the store.

While overdraft fees are a budget killer for consumers, they're a cash cow for banks. Per a MainStreet report from April citing a report from Moebs Services, banks earned a staggering $32 billion from overdraft fees in 2013. The same report also said the number of overdrafts last year stood at 7.1 per checking account, indicating how frequently consumers find themselves in an overdraft situation.

The CFPB report also said almost 75% of all overdraft fees fall on only 8% of the bank's customers.

For consumers overwhelmed with overdraft fees, don't opt-in to your bank's standard overdraft protection. It's better to have the purchase declined than to pay an overdraft fee. If you've already opted-in, contact your bank to opt-out. Some consumers have enrolled for overdraft protection without realizing it, as the forms for this process can be confusing when opening up a new checking account.

Plus, keep a closer eye on your finances. "There are so many opportunities to keep track of your purchases," says Gerri Detweiler, director of consumer education at "Download your bank's smartphone app or setup alerts should your balance fall below a certain amount."

- Written by Scott Gamm for MainStreet. Gamm is author of MORE MONEY, PLEASE