Charity door to door collections

I had a persistent lady on the phone asking me to increase my donation to Cancer Research. She was absolutely impossible to shake off, short of putting the phone down. I finally said that I didn't know how to rearrange the words any other way to tell her that I couldn't increase my contribution. I have since learned that the 'Chuggers' in the High Street who stop you, and these cold callers, work for a company, not for the charity, and they get a percentage of your contribution every time you donate - not just a one off.

A few years ago we had a young lady knock the door to ask us to contribute regularly to the Air Ambulance. I invited her in and she made it clear that she would get commission from me signing up. I joined and have been paying monthly ever since. Recently someone else knocked on the door offering the same and I said that I already contributed. They thanked me and left. The air ambulance has been used several times around here (small village) and I'm happy to support them.

She was absolutely impossible to shake off, short of putting the phone down

Why not just put the phone down and block the number then? You dont owe them anything for disturbing your day. I found that once you begin donating to a charity they can often get gredy and keep harassing you to increase your donation.