The Thirteen-cent Prexie
Third class, minimum insurance
Late in the Prexie period is was possible to pay three cents for the first two ounces of single piece third class mail plus ten cents for minimum insurance. The interval this was possible was August 1, 1958 through December 31, 1962.
Stamp dealer mail would be the most logical use of a solo Prexie for this purpose, as the dealers probably had the stamps around and felt they were not particularly valuable. The post office may have still had them as well. This cover dates from 1961.