A delivery format, not a single material
Flat-pack describes how a package is supplied before it is assembled. Folding cartons, many mailers and certain collapsible gift boxes can use different versions of this approach.
Compare the folded and assembled sizes
Ask for both dimensions and the packing arrangement. Storage and freight outcomes depend on the exact construction, order quantity and shipping method. There is no universal savings figure.
Consider the assembly step
Review the folds, closures and any adhesive steps on a sample. The right structure needs to work for the people who will pack the products.
Include the insert
An outer box may fold down while its insert arrives in a different form. Review the complete packaging set when planning storage, assembly and dispatch.