Tens of thousands of customers of state-owned Citizens Property Insurance Corp. are getting a stunning surprise in their mailboxes. It’s a letter from Citizens’ “Depopulation Unit” stating their policies have been assumed by a private-market company. Cause for celebration? Not if the private company’s estimated annual premium is higher than what the policyholder is paying Citizens. Delores Smerkers, a Davie retiree, said her Citizens policy renewed in July for $5,523 — $650 more that what she paid last year.