In our bathroom cabinet, there is only one shelf. The back is particle board so adding a shelf is questionable. I used double sided tape to secure snack-size bags to the back wall along the top to store light weight things, such as cotton balls, q-tips and band-aids.
As you see in the photo, I cut down the Q-tip box and taped it up. Put a strip of tape close to the top and one close to the bottom of the bag. Don't go clear to the ends of the bag or it won't open wide enough to get anything out.