Wouldn't a flat per diem payment make more sense then? That way you don't need the complex (and presumably pricey) expenses software, and you don't torture your employees with it when they could be doing something useful.
Unless your per diem is completely over the top, you'll need to expense flights, hotels, team dinners, client dinners, etc.
In the US, maximum allowable per diem rates vary by location and are set by the Internal Revenue Service.