I purchase zippers basically in two sizes - 22" and 9" in a multitude of colors and primarily invisible zippers. Although I do have some regular zippers left over from when I only used them.
Now once upon a time in my sewing career, I owned zippers in a large range of sizes. If a pattern called for 18" zippers - I had them. 7" or 22" or 20" I attempted to stock them all. But you know that inevitably I would go to make something and not have a zipper in the right size or the right color!
Then about a decade ago, I was at Steinlauf and Stoeller having a zipper cut and the gentlemen cutting the zipper said that I could do this at home. He then proceeded to explain to me how to do it. It's the method I've used ever since and I think different from the prevailing method...
1. Decide what size you want the zipper and place a pin across the zipper there.
2. Make sure the zipper is zipped to the top.
3. Make a bar tack across the zipper right below the pin.
4. Cut the zipper off an inch right below the thread bar.
5. Install the "new" zipper in your garment.
I've never had a zipper fail using this method and I use it often. Now that this is part of my sewing arsenal I only purchase 9" and 22" zippers in a variety of colors. It's easier to store and keep track of this way.
It's also more economical. As I said previously, this is an alternate method to shortening a zipper and I hope that you will give it a try.
...as always more later!