My ex was Korean. Her dad worked at an insurance company, and every month he got his (tiny) official salary as well as an envelop full of cash, which was 4x more money. I asked her about it and she said small companies are normally completely "off the grid" from a tax standpoint, so mid-sized companies need to do this to compete. And they all do.
She opened a bar and never, you know, registered as a business or kept books or did any of the other business things you'd need to do in the US to stay out of jail.
Her impression was tax receipts come mostly from the giant chaebol like LG and Samsung.
OK, thanks for the information! I wonder how all those small companies manage their cashflow though. Can they receive money to their bank accounts just like they would if paying taxes? It's kind of difficult to imagine they're all shuffling cash around.