On so many levels, knocking on people's doors is WRONG. Its in a private place so it can't be monitored for appropriate behaviour, it could be ANYBODY, they could say ANYTHING to persuade you, could make ANY claims, etc. At least if they were more public approaches the 'collector' might have to behave more responsibly. Why not ask people outside shops? their place of work? on the train? I know these aren't ideal but it would be harder for someone masquerading as a collector to behave innapropriately. As long as you open the door and chat to someone 'collecting for charity', the more often scammers, or worse, will call. Official-looking garb does not mean official! Don't do it!!