Show the you are in the know
Use the phrase “I understood that…” to let person you are speaking to know you’re well-informed and are suspicious they might be trick you. Don’t say, “Hey! You’re trying to cheat me!” Instead, start indirectly by stating what you know to be true: “I understood that the taxi ride to the airport was only 25 dollars.” Then give them an opportunity to respond.
Remember, you are the customer and if you have paid for a service, you should get what you want. Stay calm when you are complaining and if you don’t get what you want, ask to speak to someone more senior. Most of the time, though, a simple smile and a polite conversation using the tips above will get you exactly what you want.