From what I have seen, voluntary flying for charitable air transportation organizations like Angle Air, Mercy Flight, etc. is effectively closed to low time PPLs; especially if you don't own your own airplane. In any case, many such organizations don't pay anything or at best pay only for gas and oil (i.e., no contribution towards engine overhaul reserve and similar expenses), so it is not particularly cheap flying.
If you can fly a tailwheel airplane, glider towing is one way to build your hours for next to no money. Flying parachutists might also be available to you, although many outfits tend to be commercial operations rather than clubs, which could be problematic.
Finally,
conservation flying may be available; but again, you have to have quite of bit of experience before you will be eligible.