In Canada charity flights are covered by CAR 401.28(4).
<font face="Verdana, Arial, Helvetica" size="2">The holder of a private pilot licence may receive remuneration from a charitable, a not-for-profit or a public security organization in respect of a flight conducted by the holder as a volunteer for that organization provided that the remuneration
(a) in the case of an aircraft owned by the holder, is paid at a rate based on distance travelled or hours flown that does not exceed the total of the holder's direct operating costs and the fees charged against the aircraft in respect of the flight; or
(b) in the case of a rental aircraft, is a reimbursement that does not exceed the total of the holder's rental costs, direct operating costs and the fees charged against the aircraft in respect of the flight.</font>
In other words, it's okay for a PPL to do charity flights as long as he doesn't make a profit himself. CAR 401.28 contains similar provisions for cost-sharing with pax and for business travel- reimbursement is permitted provided that it does not exceed the cost of the aircraft.
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Per dementia ad astra