Built around the packed set
Cosmetic packaging usually holds a mix of shapes: bottles, tubes, compacts and applicators in one pouch. The internal size comes from the packed set, not from the largest single item — a pouch sized from the biggest bottle alone generally will not close over the whole kit. Decide early whether it lies flat or needs a gusset to stand, because a gusset is a different pattern rather than a larger version of a flat pouch.
How the zip performs in use
A zip contains contents properly and adds structure, which is why it is the closure specified wherever small items would otherwise escape. It is also the component most exposed to wear: the tape, the slider and the pull are three separate quality decisions, and a cheap zip undermines an otherwise good pouch faster than any other single part.
Run the zip repeatedly on a packed sample. Smoothness under load, and whether the slider wanders off the tape at the ends, are not visible in a photograph.
