okay, I would imagine you don't actually have a VISA, but if you looked at your stamp in your passport you would see it says "EXTENSION OF STAY PERMITTED UNTIL" and has a date
Also you will not be buying ANY multiple visit visa 😮 but instead you will get a "re-entry permit" (sold as a single or a multiple).
You don't mention the airport you will fly out of so I will just tell you about Suvarnabhumi. The re-entry permit counter is in International Departure 2, AFTER security screening but before passport control. When you go thru security tell them you need a re-entry permit. You don't need a photo, copies, application or anything. A single re-entry permit is 1200, and a multiple re-entry permit is 4000.
The counter is open
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and it shouldn't take longer than 15 minutes or so to get it stamped into your passport. It will be valid as long as your current extension is valid for
Looks like this