Since you ask for design advice, some design questions: You sure you want the bottom shelf to be transparent? And while I'm asking, why are any of the shelves clear? If they are covered with candy, you won't see through anyway, and if they aren't, then weight is not so much of an issue.
60" is pretty wide for unsupported shelves, generally they are no more than 36" in commercial units. And 24" is pretty deep for candy (candy boxes are around 12" deep, if memory serves, and individual bars would just get lost with the depth). Sometimes displays like this are used to showcase merchandise in front; work, sell or display on top; and have more shelves behind a false back for storage.
If you make all edges stiff, for light loads you shouldn't need to brace the middle of the shelf. It is usually prettier and easier to roll or fold the front of the shelf than to fabricate a box. Experiment first, 60" is untested territory.
Sides and backs can be grooved to hold the shelf if you don't like the look of cleats. A slender molding wrapping the side and back edges of the shelf would allow you to use shelf standards without getting into plastic fabrication, and could be very pretty.