Section 9. Estimated Additional Assessments upon Refusal to Permit Examination of Books.--The department shall have the power to make an estimated additional assessment, to which shall be added a penalty of ten per centum of the assessment, against any vendor who has filed any return as required by this act but who refuses to permit the department, or any duly authorized agent thereof, to examine his books of account and papers pertaining to the business for which the return was made. The department shall promptly thereafter give or send, by mail or otherwise, notice of such estimated additional assessment and penalty to the vendor, together with written notice of the time when and the place where the vendor may be heard on a petition for reassessment, as hereinafter provided.