Pushing and pulling which prolabs offer is done by varying the time in developer. Most prolabs will use a dip & dunk machine which will enable the operator to vary time in developer. If you have to push or pull the film will have to be done seperately hence the extra charge. Your average 1 hour photo store however will be using a roller transport processor. In these processors the only way to alter development is to alter the temperature setting in the developer, or to turn off the drive while the film is in the developer to extend the time. Neither of these is ideal, which is why these stores don't usually offer push / pull services.