A blank can include a first end panel; a first main panel joined to and extending from the first end panel; a second main panel connected to the first main panel; a second end panel joined to and extending from the second main panel; an inside end panel connected to the second main panel; and a locking tab extending from and separable from the first main panel, passable through a first opening defined between the second end panel and the inside end panel, and engageable with a locking opening separate from the first opening and defined in the blank.

An adhesive-free packaging structure includes a substrate, at least one foldable area, and at least one first connecting structure. The substrate includes a first end portion and a second end portion, and a material of the substrate includes a natural fiber. The foldable area is disposed on the substrate and includes at least one folding line. The first connecting structure is disposed on the substrate and includes a first connecting element and a second connecting element respectively disposed on the first end portion and the second end portion. When the substrate is bent along the foldable area, the first end portion and the second end portion approach each other and the first connecting element and the second connecting element are correspondingly assembled.

A packing box includes a corrugated cardboard box 3 that includes first side wall portions 7 and second side wall portions 9 and in which a vertical ridge line is formed between each of the first side wall portions 7 and each of the second side wall portions 9 with the corrugated cardboard box 3 assembled in a box shape and a reinforcing member 5 that is made of paper having a basis weight of 110 g/m.sup.2 or more and is attached to the corrugated cardboard box 3 along a ridge line portion 29 that becomes the vertical ridge line with the corrugated cardboard box 3 assembled in a box shape. The reinforcing member may have a basis weight of 270 g/m.sup.2 or more.

In one embodiment, the flat one-piece blank is provided with two extensions that extend from the side flaps to fold over the front panel to form a two pocket center seam construction that qualifies as a flat envelope under USPS regulations. In a second embodiment, the blank is provided with an extension on each side flap that has an end to be folded under and secured to the front panel to form a pair of spaced apart pockets with a third pocket therebetween. In a third embodiment, the blank is provided with an extension on only one side flap to form a pocket