Regardless of the time of day you decide to fly, the ticket price will remain around the same. Book flights from Dumaguete City to Koh Samui as you normally would.
DGT and USM are unique codes to identify airports used in the aviation industry. They are defined by IATA (International Air Transport Association) and are commonly called IATA codes. DGT to USM refers to a flight route from Dumaguete City Airport to Ko Samui Airport.