Charitable donations at the door

Just had a chap trying to get us to sign up to a weekly draw in support of the air ambulance. All very official so no problems there. I would have quite liked to give my support. However, the method was by direct debit signed up on his pad. No alternatives of signing up online, or by post. Just the pad. It is often the same at supermarkets. I have been reaching for my change, only to be told, we don't take money just direct debit donations. I just wondered what others felt. There is no way I am giving my bank details to someone at the door, I have never seen before, no matter how authentic he looks. I would be quiet happy to sign up on line where I have the time to check the security of the site and any links. Surely they must be loosing out by this single handed approach.