All licensees or permittees shall carry their license or permit certificate upon their person at all times when engaged in the activity for which the license or permit was granted; except that where such activity is conducted at a fixed place or establishment, the license or permit certificate shall be exhibited at all times in some conspicuous place in their place of business. The licensee or permittee shall exhibit the license certificate upon demand of any police officer or person representing the issuing authority, or any person invited by the licensee or permittee to do business with them. (1967 Code §51.08)