Section 30. The board of registration in pharmacy may issue to a registered pharmacist who desires to exercise the authority conferred by section twenty-nine, a certificate of fitness, stating that in the judgment of said board he is a proper person to be intrusted with such authority and that the public good will be promoted by the granting thereof. The board or the local licensing authorities may, after giving a hearing to the parties interested, revoke or suspend such certificate for any cause which they may deem proper, and such revocation or suspension shall revoke or suspend all authority conferred by section twenty-nine. Said certificate of fitness shall expire on December thirty-first of each uneven numbered year, following the date of its issue, and the fee therefor shall be twenty-five dollars for two years, or any part thereof.