By Gauss's Law:
\[ \oint \vec{E} \cdot d\vec{A} = \frac{q_{\text{in}}}{\varepsilon_0}. \]
For a spherical shell, the electric field \( E \) is constant across the surface area:
\[ E \cdot 4\pi R^2 = \frac{\sigma \cdot 4\pi R^2}{\varepsilon_0}. \]
\[ E = \frac{\sigma}{\varepsilon_0}. \]
Thus, the electric field at the surface of the spherical shell is:
\[ \boxed{\frac{\sigma}{\varepsilon_0}}. \]