Thanks for the replies everybody.
It seems like the PAX height issue would be OK, but I'm now concerned about the legality of the flight based on what people have said. I am in no way profiting from the flight and so didn't for a second think it would be illegal. I'll look into things in more detail and cancel if necessary.
For what it's worth, it seems crazy that a PPL holder can ask a PAX to contribute an equal share of the direct cost to hire an aircraft and yet the donation of a relatively small amount of money to charity could be classed as illegal.