I just spoke to Niyasha Silva (+94-773446508). She says that approval letter from Ministry of Foreign Affairs is only required for Thailand, Cambodia and Vietnam. These countries need to fill in the visa application form and send their scanned passport copies to Niyasha who will forward the details to travel agent and the agent will then arrange for the Ministry letter. All other participants will apply directly and would not be requiring approval letter from Ministry.
Hope this will help.