(2) The invoices shall be legibly written and shall be void if any corrections or erasures appear on the face thereof. Any person who alters any part of any invoice that will tend to give to the claimant an illegal gain, shall have the entire claim invalidated. The seller shall for a period of at least 18 months retain copies of all invoices and make them available to the department upon request. [Amended by 1953 c.77 §2; 1955 c.730 §10; 1957 c.209 §7]