Section 7. Additional Assessment.--If the department is not satisfied with the return and payment of tax made by any vendor under the provisions of this act, it is hereby authorized and empowered to make an additional assessment of the tax due by such taxpayer, based upon the facts contained in the return or upon any information within its possession or that shall come into its possession. Promptly after the date of such additional assessment, the department shall give or send, by mail or otherwise, a notice thereof 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.