Well, I did ask some of the hawkers out there. They specifically told me that they try to bypass tax as much as possible. For example, even for touch and go transactions, they try to use touch and go QR code that is not link to them.

Also there is another korean restaurant near my house. If I pay by cc, I need to pay 10% extra. I am pretty sure this 10% is not just covering cc charges but taxes as well.

And also they do not accept tng payments as well. I mean it's obvious why they dun do that.

I would say they are smart. First of all, they still declare taxes to the government as there are going to be many people who pays using CC. But at the same time, they can bypass taxes for most of their other non cc transactions as well.

One of the reason why governments around the world wants to go cashless is because once everything goes cashless, there is no way to bypass taxes anymore. As long as cash is being used, you can easily bypass taxes.

I think what they do is very fair. Their price is pretty cheap as well.

In fact just because of this, I visit their shop even more as compared to shops which charge so high just because they give you the convenience of using a cc.

Would not say is "surchage".
They just waive the service charge  if you pay by cash that's all.