A flexible sidewall of a package has an inner surface that directly interfaces with product that is contained within an enclosed volume of the package. A sealed closure of the sidewall contains a material that forms a mark on the package, the mark being visible from an outer surface of the package. A production line can form a plurality of the packages, in sequence, after the mark is formed on the sidewall of each package. An outer package of a packaged product can contain a quantity of product and the aforementioned package, such that the outer surface of the package interfaces with the quantity of product. The material forming the mark on the package, being contained within the sealed closure thereof, is not exposed to the product that is contained therein, nor to the quantity of product contained in the outer package.