I don't think the decision was targeted at shopkeepers in particular. I've had a look at my own VAT envelope (I am self-employed) and yes, it requires a stamp. So, yes, they are clawing more back. I reckon if Mike leaves the stamp off he will get away with it on the basis that a) they want the money and b) they might assume you didn't notice this time. I don't think it will work for ever, though.