Supplementary material for the paper "Relativistic, model-independent determination of electromagnetic finite-size effects beyond the point-like approximation" arXiv: 2109.05002 [hep-lat] It includes: FVE_calculation.nb: Mathematica notebook where the major analytic results presented in the paper are derived; fvtools.wl: Mathematica package for the evaluation of loop-integrals and of the finite-volume coefficients entering electromagnetic finite-volume effects. Acknowledgments A.P. would like to specially thank Martin Savage for his precious input in the early stages of this project. A.P. additionally thanks Zohreh Davoudi as well as members of the RBC-UKQCD collaboration for useful discussions. A.P. would like to thank both the Institute of Nuclear Theory at the University of Washington and the Albert Einstein Center for Fundamental Physics at the University of Bern for their warm hospitality during visits that played a crucial role for the completion of this work. Similarly, N. H.-T. wishes to thank the Higgs Centre for Theoretical Physics at the University of Edinburgh for hospitality during visits to work on this project. Important parts of this work have been completed during these visits. M.T.H. would like to thank Tim Harris for useful discussions and all authors would like to warmly thank Nazario Tantalo for his critical read of the manuscript before its first release. M.D.C., M.T.H., and A.P. are supported in part by UK STFC grant ST/P000630/1. Additionally M.T.H. is supported by UKRI Future Leader Fellowship MR/T019956/1. A.P. additionally received funding from the European Research Council (ERC) under the European Union���s Horizon 2020 research and innovation programme under grant agreements No 757646 & 813942. N. H.-T. is funded by the Albert Einstein Center for Fundamental Physics at the University of Bern.

This work has been also supported by UKRI Future Leader Fellowship MR/T019956/1 ("New Physics meets the Strong Force")