Let Q denote a smooth manifold acted upon smoothly by a Lie group G. The G-action lifts to an action on the total space T*Q of the cotangent bundle of Q and hence on the standard symplectic Poisson algebra of smooth functions on T*Q. The Poisson algebra of G-invariant functions on T*Q yields a Poisson structure on the space (T*Q)/G of G-orbits. We relate this Poisson algebra with extensions of Lie-Rinehart algebras and derive an explicit formula for this Poisson structure in terms of differentials. We then show, for the particular case where the G-action on Q is principal, how an explicit description of the Poisson algebra derived in the literature by an ad hoc construction is essentially a special case of the formula for the corresponding extension of Lie-Rinehart algebras. By means of various examples, we also show that this kind of description breaks down when the G-action does not define a principal bundle.