The same approach applies when calculating the force on a charge element dq at the surface of a spherical shell of charge, due to all other charge elements making up the shell. Setting up the integral explicitly and taking the limit as the contributions from the shell include all elements except the one excluded, we find a field at the boundary of magnitude $$\sigma/(2\epsilon_0)$$, or half what one would obtain if using the field just outside the shell, due to the entire shell.