I am arriving at Bangkok at 6pm. What would be the least expensive and easy way to reach Koh Samui (no flights please
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The last sleeper train to Surat Thani leaves Hualamphong station at 7:30pm but doubtful you'd make it in time. There are overnight trains at 10:50pm but just reclining seats no beds. Otherwise get to the new southern bus terminal and hope for a seat on whatever's heading south. From Surat Thani (Phunphin) station to Don Sak pier is about 80km but transport with connecting ferry service is abundant. About 500 baht for bus/ferry to Samui.
Alternatively, you could look into Lomprayah. It is possible to make it by 9. Otherwise, you could go in the morning!