Security Pacific Merchant Banking Group announced the underwriting of a $375-million tax-exempt bond issue for the regents of the University of California, which, pending the new tax laws, may be the last of its kind . . . People Express said it is offering half-price fares on all domestic flights between Oct. 2 and Nov. 19, provided the tickets are purchased by Oct. 1 . . . Standard & Poor's said it has placed some Allied Supermarkets debt on creditwatch with a possibility of downgrading because of Allied's agreement to merge with Vons Cos.