Recently I even needed my rims redone on my car, went to a big autoshop in my town, the owner came out with me to look at the car and went "mate it's going to be £500, cash only, you know how it is with the taxes. Or I can give you an invoice but it's going to be £600".
I literally turned around and left. Yes, they are crooks and fraudsters - but in my experience it's completely normalized. If you can get away with it, they will do it, and since everyone seems to be getting away with it, they do exactly that.