Decorated-box-diagram-contributions to Bhabha scattering. (I)

We evaluate, in the high-energy limit, $s\gg|t|\gg m^2\gg\lambda^2$, the sum of amplitudes corresponding to a class of Feynman diagrams describing two-loop virtual photonic corrections to Bhabha scattering. The diagrams considered are box and crossed box diagrams with an extra photon decorating one of the fermion lines. The mathematical method employed is that of Mellin transforms. In the eikonal approximation, this sum of two-loop amplitudes has previously been evaluated, and found to be equal to the sum of the box and crossed box amplitudes, multiplied by the electric form factor of the electron. We obtain a similar factorization, but with the form factor replaced by another expression involving the logarithms $\log(\lambda^2/m^2)$ and $\log(\lambda^2/|t|)$.