More often than not,their threats are meant to intimidate. If you hold your ground,most likely they leave you. But you better be on the 'legal" side of things.
It happened to me last year, some kra lady came slapping me with 500k fine because i made a sale "without" charging VAT. I told her i didn't have the bribe she was asking for, 250k but i promised to do so the following day. Come the following day, before she could even call me, i called her and told her i was at Times Tower and i wanted to meet her . Her response was she was out,she would get back to me in a few minutes. I never saw her again even though she tried calling once. Not unless they have concrete evidence, the thing of "their word against yours" can't take them far.
If just saying someone made a sale without issuing etr receipt makes one legally liable, i guess all businesses would at the mercy of vat officials.