Shame, mistakes happen. it is good to keep revising and viewing your listings and all areas of specifications periodically. Airbnb keep changing things. Sometimes i do not even know how my settings change themselves!!!.. like when i put minium 2 days booking it reverts back to one day.. i check all things often.
Ok so you mistaking left it at £7 and someone thought it was their lucky day and booked for 3 months????
Email Airbnb and explain a simple mistake and say it was a genuine mistake as clearly your room is not so cheap and you would be at lost to honour the £7 per day.
I think airbnb try to eradicate those who cancel casually and often etc as this would ruin their and our business of providing accomdation. Travellers do not like to be let down like this. But i feel you have a genuine case of error and should request and politely ask that they give you this one chance to be a little more diligent in the future. I hope it works, i think they should be fair on this one.
Mala
