You swipe your debit card and enter your PIN, only to discover your debit card has been declined for insufficient funds. It’s awkward and embarrassing. The solution the banking industry has devised is overdraft protection, a service that allows users to withdraw more money than their accounts contain – for a fee. The median charge is generally around $30 to $35.

Such protection may seem like a consumer-friendly idea, but its implementation has not always played out that way. Earlier this month, global banking giant Santander was fined $10 million for using illegal and deceptive practices to enroll customers in overdraft protection services. And a 2013 report from the Consumer Financial Protection Bureau found that overdraft fees cost American consumers $32 billion in 2012, up from $20 billion in 1990.

Consumers who want to avoid paying a share of that sum, therefore, should make sure they know the basics of how overdraft protection works. Keep in mind that a 2010 regulation prevents banks from charging overdraft fees for ATM and debit card transactions to consumers who have not actively opted in to the service. Yet, a 2014 report by the Pew Charitable Trusts found that 53 percent of customers with overdraft protection don’t remember deciding to enroll, suggesting that it may be easy for consumers to overlook the relevant information among all the paperwork involved in opening a bank account.

If you do decide to consider overdraft protection, make sure you know how your bank structures its service.

First, ask for a complete fee schedule. In addition to the initial fee for overdrawing the account, there may be other, less obvious charges; the Pew report found that the average overdraft event costs a consumer $69. For example, the bank may impose additional fees if money isn’t deposited to cover the overdraft within a certain timeframe. Some will allow an unlimited number of overdrafts; others put a cap on fees.

Also find out how the bank decides what order to process transactions in. Avoid signing up if your bank processes the largest transaction from a given day first. This practice increases the chances of causing an overdraft. Then, once your account balance dips below zero, each additional charge incurs another fee.

The Consumer Financial Protection Bureau has found that overdraft practices vary widely from bank to bank, so, above all, research the details before you make any overdraft decisions.

Have a consumer question or complaint? Reach Sarah Shemkus at seshemkus@gmail.com.