When in package form, and when packed, possessed, offered, or exposed for sale or sold at retail, wheat flour, whole wheat flour, graham flour, self-rising wheat flour, phosphated wheat flour, bromated flour, enriched flour, enriched self-rising flour, enriched bromated flour, corn flour, cornmeal, and hominy grits shall be packaged only in units of 2, 5, 10, 25, 50, or 100 pounds, or multiples of 100 pounds, avoirdupois weight. However, this section does not apply to prepared ready-mixed flours and meals, and special cake flours in packages the net contents of which are less than 5 pounds.