Baey's elaboration of the explanation given by the store owner is all over the shop and sounds more like a deliberate attempt to obfuscate what was really just a simple scenario.
The first five paragraphs add no value and even when the explanation was finally given from paragraph 6, they all pointed to different directions without any clear line of thought.
First it was claimed that the Nasi Padang + Bandung was originally $4 but a staff gave him a $1 discount out of goodwill to recognize his work so he was charged $3 instead. Baey claims he eventually cited $2.50 because he estimated the Bandung was worth $0.50. This sentence already sounds silly on its face, Bandung is now going for $1.50 to $2.00 these days, what $0.50 is he talking about? Either he is really ignorant (which would further attest to his detachment from commoners), or he knows that if he subtracted $1.50 for the Bandung, he would be left with only $1.50 for the Nasi Padang which would be too ridiculous to pass the laugh test.
Subsequently no head no tail the story changes to how there was an ongoing $2.50 promotion for elderly and students which included a drink. What does that have to do with Baey's order? He is neither a senior citizen nor student anyway. I thought the original explanation is that he was given a discount out of goodwill for his hard work? How on earth does this promotion have anything to do with the matter at hand other than the fact this magic $2.50 number happens to match what Baey said he paid for the Nasi Padang which in turn was calculated on a nonsensical $0.50 Bandung assumption in the first place?
As for whether the $6 included undeclared items, it's basically the store owner's words against the FB poster so I will reserve judgement.
The rest of the rah rah like how the promotion is being extended to another 100 patrons now, how Baey's activists are helping to share etc. is quite simply filler information that has nothing to do with explaining the discrepancy other than to shout "I'm a good guy".
The final paragraph on how he was charged $3.70 only for mutton, veggie, rice and egg is actually the real gem hidden in this wall of text. If he wasn't that detached from actual market prices on the ground, he would have known that it is almost impossible for such a combination to sell at $3.70 under normal circumstance. Granted that sometimes if the stall owner don't declare a discount, it would be difficult for him to reject and "force" the owner to admit, but the fact that he can still in all seriousness share this publicly just goes to show Baey is likely so encapsulated in his reality bubble that he sincerely thinks $3.70 for this combination is "normal".