DALLAS (AP) -- Southwest Airlines said Sunday it has begun filing refunds to customers who were accidentally billed multiple times for a single flight, after a half-price online ticket promotion backfired.

The special sale designed to celebrate Southwest's reaching the three million mark in Facebook fans was supposed to give customers who booked flights on Friday a 50 percent discount on certain fares.

Instead, hundreds of frustrated would-be fliers wound up posting stories on Facebook about how their credit and debit cards were being repeatedly charged, in some cases 20 or more times for a single flight. They also tweeted and blogged about the related financial hassles, which ranged from drained checking accounts and bounced checks to overdrawn credit limits and canceled bank cards.

The company said Sunday it has identified all customers impacted by the problem and has begun filing refunds to the affected bank accounts.

"These refunds are currently being processed, but timing will vary depending on the individual bank," Southwest said in a statement to its website. "If a refund has not yet posted to your account, we recommend contacting your bank."

The company, which described its response as an "all hands on deck approach," said it will also pay customers for any overdraft fees caused by the additional charges.

Southwest Airlines is based in Dallas, Texas and offers flights to more than 70 U.S. cities