I think of the boxes as functional sculpture.  Working with collage on a three dimensional surface requires consideration of how all sides (including the inside) work together.  I use heavy layering of color and texture to build up the surface, sometimes molding the paper to create a certain shape, line or to add dimension.  Fragments of shells, beads, or dried plants are often worked into the design.  For me, the boxes (many of which have companion jewelry) are magical places to hold tiny treasures.