Why ZIP codes have 5 numbers – and what they each mean

"The ZIP+4 code was implemented when it became difficult to sort mail based only on traditional ZIP codes, but it never quite caught on."

This is totally false. Zip+4 addressing (and, for bulk mail, presorting by Zip+4) is required for the best mail rates. Zip+4 is generally not written in by the consumer (I do, but I'm a geek who's done mailhouse work), but it's endemic otherwise.

I bet a fair share of any reader's outbound mail here uses zip+4 as well - just look at the return envelopes/labels for bills, etc. that are done via mail.