Determining the distance between two ZIP codes (alternatives to mapdist)

Question

I want to calculate the distance between approx. 100,000 different ZIP codes. I know about the mapdist function in the ggmap package

Answer 1

This might be trivial, but one completely free option is to use Census ZCTA geography data to get co-ordinates for each zip code, and then calculate Haversine distances (or some similar distance metric) between coordinates.