  We study the shell-model formalism to include the isospin-symmetry-breaking
correction ($\delta_{C}$) to nuclear matrix element of superallowed $0^+\to
0^+$ Fermi $\beta$ decays of $T=1$ nuclei. Based on a perturbation expansion in
a small quantity, such as the deviation of the overlap integral between proton
and neutron radial wave functions from unity or of the transition density from
its isospin-symmetry value, we derive that $\delta_C$ can be obtained as a sum
of six terms, including two leading order (LO) terms, two next-to-leading order
(NLO) terms, one next-to-next-to-leading order (NNLO) term and one
next-to-next-to-next-to-leading order (NNNLO) term. The first two terms have
been considered in a series of shell-model calculations of Towner and Hardy as
well as in the recent calculation of the present authors, while the remaining
four terms are usually neglected. A numerical calculation has been carried out
for 13 transitions in the $p$, $sd$ and the lower part of $pf$ shells. Our
results indicate that the magnitude of the sum of all higher order terms is of
the order of $10^{-3}$ %. This number is well below typical theoretical errors
quantified within the shell model with Woods-Saxon radial wave functions.
