Is there a prefix or adjective that means "one and a half", as "tri-" or "triple" is for "three"?

The exact usage I have is to describe "18" in terms of a dozen. Where I live they've started making 18-egg cartons, and the local grocer was wondering what name to give them in his computerised cash register, hoping for a single word.

Even slang, informal or invented term would do if there isn't such a term already.