Like said above, CYLOR.
I’m not a Rav, but I’ll add my 2 cents of analysis.
Removing a tag (the kind which is never worn) from a new garment is preparing the garment for use. That is Makeh DePatish, putting a finishing touch (see Shabbat 75b). Possibly a Torah level violation of Shabbat.
Tearing open a plastic bag to get what is inside is at most a rabbinical prohibition of mekalkel, destruction. All prohibited-from-the-Torah acts on Shabbat are constructive, except for carrying (which is not constructive but forbidden by the Torah). Purely destructive acts are prohibited rabbinically.
I don’t think that wearing a price tag on a garment is considered a violation of a Torah level melacha. That’s not the usual way of carrying. Could be rabbinically forbidden.
AFAIK, the first Rav is more correct, but as said before consult a posek.