Ciao Siu18769

Welcome to OSM, it's nice that you're here and help to update the map data.

The `levels` tag is undocumented in the OSM Wiki.
In my opinion it is a nice value to record for shops though!
I would go for how the level tag is supposed to be used for multiple levels as specified in the Wiki: https://wiki.openstreetmap.org/wiki/Key:level#Multiple_values

This works for multilevel shops like Ochsner Sport (https://www.openstreetmap.org/node/2725160807) on one hand where I would tag it like `level=-1;3` since it also has an underground level.
Mishio (https://www.openstreetmap.org/node/313812140) on the other hand is already tagged with `level=1`, since it is on the first floor.

Greetings and happy mapping,
habi

PS: Wir können gerne auch auf Deutsch schreiben.