Regardless of the time of day you decide to fly, the ticket price will remain around the same. Book flights from Jakarta to Saint Lucia as you normally would.
These are just IATA (International Air Transport Association) codes. They are used in aviation to identify and differentiate airports. That’s why a Jakarta Soekarno-Hatta Intl Airport to Castries Hewanorra Airport flight would often be abbreviated as a CGK to UVF flight. Castries George F. L. Charle Airport (SLU) are some additional IATA codes for airports related to your trip.