I printed my shelving labels using eSUN Bright White PLA+, which gives enough flexibility to snap in and secure very well to the front of the shelves, although other semi-flexible plastics such as PETG should also work just fine. Pretty much anything which is not particularly brittle. As for printing settings, I printed these with fairly thick layers and printed the "horizontal" variant so that the side on the glass faces forward. This way, the top/bottom infill pattern would show, and the surface would be nearly as flat as the glass. However, I've also attached a "vertical" variant if you'd rather print in that direction - it is the same except for the orientation.