  A simple method to compute QED bound state properties is presented, in which
binding energy effects are treated non-perturbatively. It is shown that to take
the effects of all ladder Coulomb photon exchanges into account, one can simply
perform the derivative of standard QED amplitudes with respect to the external
momentum. For example, the derivative of the light-by-light scattering
amplitude gives an amplitude for orthopositronium decay to three photons where
any number of Coulomb photon exchanges between the e-e+ is included.
  Various applications are presented. From them, it is shown that binding
energy must be treated non-perturbatively in order to preserve the analyticity
of positronium decay amplitudes.
  Interesting perspectives for quarkonium physics are briefly sketched.
