It seems that the slot that I'm interacting with is not the one I'm pointing at, but rather the one that my line of sight intersects with on the block of the shelf.
I realize that getting the math of where the user is pointing at for a plane in the middle of the block. Thus I recommend you simply move the shelf to be rendered in the front half of the block rather than the back half. Thus a player will look at the front face of the block when indicating where he wants to deposit an item.
It seems that the slot that I'm interacting with is not the one I'm pointing at, but rather the one that my line of sight intersects with on the block of the shelf. I realize that getting the math of where the user is pointing at for a plane in the middle of the block. Thus I recommend you simply move the shelf to be rendered in the front half of the block rather than the back half. Thus a player will look at the front face of the block when indicating where he wants to deposit an item.