Tell me: What do these maps exactly diffuse, normal, specular (textures)

Basically, your glass item should have the PhongAlpha shader and your alpha channel on your diffuse should be black, for full transparency, grey for partial transparency. The darker your texture's alpha channel, the more transparent your object will be.

This tutorial is helpful, as well: https://sims4studio.com/thread/1157/objects-glass-balloon-effect?page=1