The Dept. of Transportation rules about airfare transparency don’t just apply to carriers’ websites and ads, but also to their Twitter feed. Just ask Spirit Airlines, which was slapped with a $50,000 fine for Tweets touting its $9 airfares.

According to DOT rules, any advertising that includes a price for air transportation must state the full price to be paid by the consumer, including all carrier-imposed surcharges.

But this past June, Spirit used its Twitter account to tout $9 each-way fares, except that when you clicked on the link, that’s when you found out that you also had to pay additional taxes and fees, and that you had to buy a round-trip ticket. You had to click a a second link to see all the fees, etc.

“Consumers have a right to know the full price they will be paying when they buy an airline ticket,” said U.S. Transportation Secretary Ray LaHood. “We expect airlines to treat their passengers fairly, and we will take enforcement action when they violate our price advertising rules.”

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