The U.S. Transportation Department says United Airlines must pay a $30,000 fine for failing to include a federal tax when it quoted some airfares on its Web site.

The government said on Friday that United omitted a 7.5 percent federal excise tax from some search results on its Web site. The tax was left out of the fare for about two-and-a-half days.

Government rules require most mandatory taxes and fees to be included in the quoted price of the ticket.

In August, the Department of Transportation fined United $75,000 for other advertising violations. United only had to pay half, as long as it had no more violations. Now, the agency says United will have to pay the other half of that civil penalty, or $37,500.

The company is a unit of Chicago-based UAL Corp.