What is Neptune's perihelion and aphelion?
4444.5 Million km and 4545.7 Million KM
Semi-major axis of Neptune's orbit a = 4495.1 Million km. Eccentricity of the orbit e = 0.011257, nearly.
Reference: NASA Planetary Fact Sheet.
Some other sources might give data differing slightly, in the least significant digits (lsd)..