Technically, the only difference between a bumper sticker and a decal is one has removable adhesive and the other doesn't. We screen print BS on General Formulations matt vinyl with removable adhesive and print door decals with the same vinyl with "permanent" adhesive.
If your digitally printing go for General Formulations Metro Mark vinyl which is made for easy, clean removability. For smaller decals that clients usually put on car windows I like Oracal 3164 because of the 4 mill thickness which makes application without premask easier. It's permanent adhesive but on glass they can easily be removed with a razor blade anyhow.