  Using the standard Bardeen-Cooper-Schrieffer (BCS) theory, we revise
microscopic derivation of the superconductor-insulator boundary conditions for
the Ginzburg-Landau (GL) model. We obtain a negative contribution to free
energy in the form of surface integral. Boundary conditions for the
conventional superconductor have the form $\textbf{n} \cdot \nabla \psi =
\text{const} \psi$. These are shown to follow from considering the order
parameter reflected in the boundary. The boundary conditions are also derived
for more general GL models with higher-order derivatives and pair-density-wave
states. It shows that the boundary states with higher critical temperature and
the boundary gap enhancement, found recently in BCS theory, are also present in
microscopically-derived GL theory. In the case of an applied external field, we
show that the third critical magnetic-field value $H_{c3}$ is higher than what
follows from the de Gennes boundary conditions and is also significant in
type-I regime.
