Yeah, I always thought that was a bit of an aCkTcHUuaLly type of point. Okay, okay, it's about routes. But, when you can draw perfectly mutually exclusive convex hulls around the unions of the lots receiving mail for a given zip code, then you can draw zip code polygons.

I don't think zip codes in practice can be both convex & mutually exclusive.

Imagine a river that divides two zip codes, with river-front property on both sides. If that river has a bend in it (-∿-) that's greater than the width of the river+property, the polygon bounding one zip code would cut through the other, or the polygon is non-convex where it bends along the river.