I have all of my bins numbered from 100 up. I have different sizes in different places but the numbers are in order. When I only carry 5 of an item I find the appropriate size bin and put them in it. How do I find it when I want it? I have a binder filled with baseball card holders in it. I cut out the picture of the figure from the catalog, staple it to the back of a business card size piece of card stock and write the bin number on the top right. I file the cards in the book alphabetically.
When I am looking for a part, if it isn't in the book, it's not on the shelves. I can use the bins most efficiently for space and I can move things around any time I want. On the cards I also have the part number so I can re-order easily. I also put the price group.

I also use the baseball card holders for mylars. So many times you only keep a few of each. This way I can group them by category and let the customer pick the one they want. I just cut them individually and use both sides of the pockets.