Flat-profile construction
The flap gives this style a flat profile that stacks and ships well, and a clean opening moment at the counter. Flap depth is specified from packed thickness, so send the depth of what goes inside rather than only its width and height.
How the envelope flap behaves
A flap gives a flat profile and a defined opening moment, which is why it is common in jewelry and accessory packaging. The detail that decides whether the sample is accepted is flap depth: the flap has to lie flat when the pouch is full, not only when it is empty. That depth is specified from packed thickness, and getting it wrong is the single most common reason an envelope-style sample is remade.
Approve it packed. An empty flap pouch tells you nothing about how the flap sits in use.
