Large Picture Boxes (PIC-K)

We ordered the small, 28x 37x4 frame boxes and easy to assemble and very satisfied with the product so ordered the pack of 3 large frame boxes (40x60x3.5) and thought they sent the wrong size since we got 12 20 inch boxes. I called and was informed you need to use 4 pieces to make one large box and this information was on website when I ordered it. There is NO where on website that states "assemble 4 pieces to make ONE box" and it was not even in the packaged delivered. There is a picture, but did not state "this is to assemble".This was frustrating since have to use a lot more tape and time to create your own box and either have a clumpsy-extra cardboard box or have to cut it.