It’s best to take plane to Bhutan. At present, only Kathmandu in Nepal, New Delhi, Kolkata in India, Dhaka in Bangladesh, Yangon in Myanmar, and Bangkok in Thailand have operated direct flights to Bhutan.
Bhutan doesn’t strictly follow the rules of the international aviation. Flights in and out of Bhutan are exclusively operated by Royal Bhutan Airlines, and other airlines are not allowed to fly the Bhutan route. Even if it is monopolized, objectively speaking, the airline is never inferior to its international counterparts in safety and service.
Paro Airport is the only international airport in Bhutan and there is no international airport in Thimphu, the capital city of Bhutan. As the flights are monopolized by the Royal Bhutan Airlines, air fares cost higher, and no discount is available. The departure tax at Paro Airport is around $13.