Thanks flossy63, I'll get in touch with them.

Wonderful, thank you Lucy, I have filled out the form that flossy63 suggested.  

Oh yes, definitely not interested in registering as a charity.  Your suggestion was what we thought of initially, but wondered if people would worry that we were just trying to boost the price with no intention of donating (though we'd provide a receipt of the donation).  Would be awesome if Trade Me had a function like described.