Gordian Software, which builds tech that helps airlines and travel websites upsell customers with add-on features, raised $25 million. The Bellevue, Wash.-based startup was founded in 2017 by Stephen Grabowski and Joe Sarre. They worked together at Skyscanner, a travel search engine, then teamed up to build Gordian. The startup provides an application programming interface, or API, that plugs into an existing ticketing system to upsell ancillary products, such as baggage or seat selection.