  Recently, a framework has been developed to study form factors of two-hadron
states probed by an external current. The method is based on relating
finite-volume matrix elements, computed using numerical lattice QCD, to the
corresponding infinite-volume observables. As the formalism is complicated, it
is important to provide non-trivial checks on the final results and also to
explore limiting cases in which more straightforward predications may be
extracted. In this work we provide examples on both fronts. First, we show
that, in the case of a conserved vector current, the formalism ensures that the
finite-volume matrix element of the conserved charge is volume-independent and
equal to the total charge of the two-particle state. Second, we study the
implications for a two-particle bound state. We demonstrate that the
infinite-volume limit reproduces the expected matrix element and derive the
leading finite-volume corrections to this result for a scalar current. Finally,
we provide numerical estimates for the expected size of volume effects in
future lattice QCD calculations of the deuteron's scalar charge. We find that
these effects completely dominate the infinite-volume result for realistic
lattice volumes and that applying the present formalism, to analytically remove
an infinite-series of leading volume corrections, is crucial to reliably
extract the infinite-volume charge of the state.
