There's no such thing as a 30 day VOA <- Visa On Arrival 😮
If you showed up here from Japan without having bought a visa to thailand you were stamped in FREE for 30 days on what's called a "visa exempt entry".
You can extend that one time for 30 more days at the immigration office that serves the area where you stay for 1900baht.
In Bangkok on a visa exempt entry stamp you go to IT Square Laksi Plaza for your extension;
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