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We assume that surface forces are applied on the shell front face ($\xi_{3}=+1$ ). Therefore, the external work of surface forces has the form $$\begin{equation} \delta W_{ext}=\int_{\partial v}\delta\mathbf{u}\left(\xi_{1},\xi_{2},+1\right)\cdot\mathbf{t}\,da=\sum_{a}\delta\mathbf{u}_{a}\cdot\int_{\partial v}M_{a}\left(\xi_{1},\xi_{2}\right)\mathbf{t}\,da\label{eq:fbs-ext-work} \end{equation}$$ In other words, the treatment of surface forces on a shell becomes identical to the treatment of surface forces on the face of a solid. No special treatment is needed.