"Standing / Non-reserved Seat" is the cheapest fare on Korail:
But how can I purchase a "Standing / Non-reserved Seat" Korail train ticket online?
I only see two purchase options (first class and economy) on Lets Korial:
while clicking on fare:
shows a third fare category "Standing / Non-reserved Seat" (shown in the first screenshot).
E.g. the SNCF in France does allow online booking for non-reserved seats ("TGV : place(s) assise(s) non garantie(s)"), so it's conceivable that Korail allows the same.