Booking a Druk Air ticket is always an interesting challenge. Once upon a time you had no choice but to use an agent in Bhutan. Then came online reservations using Druk Air's website, with its own idiosyncrasies. But recently I noticed that Cleartrip, the Indian online booking engine, shows Druk Air flights on its search results.
Now that's new. Searching for a flight from Paro to Bagdogra I found a price that is higher than the official Druk Air rate. Also, flights seemed to be available on all days of the week, which is strange. All in all, it seems all this is new and still not ironed out, so I wouldn't recommend being the first to try it out. But with Drukair facing competition in the future, uploading their flights into international booking engines is the way to go.