The powder was discovered by a postal worker late on Monday.
Hustings will be held in early August, followed by a postal vote of members.
The new leader, elected by a postal ballot of Ukip members, will be announced on 28 November.
There was "Touched by an Angel," and now comes "Signed, Sealed, Delivered," so named, presumably, because someone realized that "Touched by a Postal Carrier" might send the wrong message.
Oklahoma is the place where "going postal" became an unfortunate American cliché a generation ago after 14 co-workers were shot dead by a postal worker who then killed himself.
A similar contest in Totnes in August, which was decided by a postal vote rather than an evening of deliberation in a golf club, attracted one in four of the local electorate.
A postcard featuring a rear view of nine naked men, meant to advertise a program of short films at the Brooklyn Academy of Music, was rejected by a postal official in Staten Island.
The 20-page criminal complaint in the case, sworn out by a postal inspector, Eleanor Berry, accuses Mr. Esmerian of a range of schemes and transactions, from the loans and surreptitious sales of various multimillion-dollar pieces of jewelry and collectibles to fraud in connection with a transaction involving his apartment.
A trick where a pickpocket removed not only wallet, but also braces, was made wonderful by the indifference of everyone standing by; and Mr Routh's journey as registered "livestock" through the post from Sheepwash in Devon to Fleet Street, accompanied, as required, by a postal official, was deeply enhanced by the official's polite silence all the way.
And her love songs often come with a twist, like "E.C.T.," in which a love letter is opened by a postal clerk who turns it into a hit song, or "Beija Eu" ("Kiss Me"), which toys with its rhyming Portuguese syllables while its meter shifts between 3/4 and 4/4.
