Google dominates the online search market, with at least three-quarters of the market. People use the search engine to research trips, so for at least a decade, online travel agents have refined their websites with trustworthy content and easy booking tools to show up high in Google results.

This search engine optimization (SEO), worked well until about five years ago. Around that time, Google began placing more ads on the top of search results, pushing down the free listings. The internet giant also built new travel search tools, which were mostly paid listings.

This means online travel agents must now pay billions of dollars each year to Google to ensure they show up high in search results and get clicks from travel planners.

Related: Google pushes travel websites down in free search results