I've sometimes been able to get a cash discount by asking for it. This won't work with corporate places, obviously, but sole-proprietor sorts of shops - like my local car mechanic, and a print shop I used to use occasionally - have been receptive. I think they discounted by ~5% in each case. Did I suspect they're keeping the cash off-book? Of course. Did I care? Not at all. I don't expect their big-business competitors are paying tax, either.