  This note revolves on the free Dirac operator in $\mathbb{R}^3$ and its
$\delta$-shell interaction with electrostatic potentials supported on a sphere.
On one hand, we characterize the eigenstates of those couplings by finding
sharp constants and minimizers of some precise inequalities related to an
uncertainty principle. On the other hand, we prove that the domains given by
Dittrich, Exner and \v{S}eba [Dirac operators with a spherically symmetric
$\delta$-shell interaction, J. Math. Phys. 30.12 (1989), 2875-2882] and by
Arrizabalaga, Mas and Vega [Shell interactions for Dirac operators, J. Math.
Pures et Appl. 102.4 (2014), 617-639] for the realization of an electrostatic
spherical shell interaction coincide. Finally, we explore the spectral relation
between the shell interaction and its approximation by short range potentials
with shrinking support, improving previous results in the spherical case.
