What if you forget to write the zip code on a piece of mail?

I dropped three postcards in a rush into a blue mail pick-up bin in Union Station in Los Angeles on a Monday evening in December. In my hurry, I had forgotten to write the zip code of the delivery address on two of those postcards (I wrote the addresses down from memory, and meant to double-check the zip codes of two of them later). The sole postcard with the complete address arrived in rural North Carolina on Saturday. A postcard missing its zip code arrived on the Eastern Shore of Maryland on the following Thursday. And the last postcard was also delivered around the same time, zip code-less, to Houston. So forgetting the zip code adds about 5 days or so to the journey of your mail.