I think I may be able to help you here. You do not have very many listings in your shop, so I think adding custom listings could be a good way to go for you to expand your offerings - as long as you have photos of cases you have made in the past. Showing a big variety of colors and buttons, etc will help your customers envision the case they want for themselves. You have to remember though, that someone may order a color that is not popular and then you will be left with extra yarn that sits around unused for a long time. There is a certain amount of waste that goes along with taking custom orders and allowing customers to choose any color.
I think adding initials would be a good way to customize an eyeglasses case. Otherwise, I am not sure why someone would want a custom case as opposed to seeing one you made and liking that one and being willing to wait a few days for a made-to-order item.
I hope that makes sense.