Even when there is a tiny sign at checkout, its not enough (in my view) because I've already wasted time and shopped by then and the merchant is inviting a really negative response when he 'springs' this on me.
I've no problem with leaving my shopping behind and not going through with a sale in principle, but being generally time poor as I am I shop with purpose (haha) and having shopped need to get those purchases somewhere so they can be used, leaving the transaction behind and shopping somewhere else is pretty inconvenient ... no to mention that giving back ones business lunch is a tad difficult too (!).
Anyway, long rant shortened, if we are to continue to have legal surcharging I'd be in favour of government, through its various retail oversight bodies, making it compulsory to ensure there is adequate signage so customers don't get trapped at the till.