The IRS has standard per diem rates different from the GSA/DOD/State Department rates. And the tax write off of the difference between what you were paid and the GSA et al. rates went away with tax reform a few years ago. The IRS rates for transportation workers are $66 a day domestically and $71 internationally. Atlas can’t even figure that out so they just withhold on what you’re paid above the $71 a day/$2.91 an hour rate even if you’re domestic.