Relativistic correction to Coulomb Potential in SQED

[Seriva]

Homework Statement

Obtain the relativictic lower order correction to the Coulombian potential

Relevant Equations

Those ones that are in the appended photo

I have derived the Coulombian potential as an effective potential between two spinless charged particle taking the non-relativitic approach on the scattering amplitude obtained in terms of the Feynman rules in SQED.
The scattering amplitudes are:

I'm using the gauge in which xi = 1.
How could I obtain the relativistic lower order correction of the Coulombian potential using those amplitudes??