This happened to me once with a bag and I couldn't get back to the shop in time because I needed it the same night. I called and the manager told me to hold it tightly at the edges, and get a hammer and bang the middle of the tag and the edges and then remove it with a plyer - a little extreme but it worked and there was a tiny (literally, barely even noticeable) hole. I'm not necessarily advising you to do this, as different stores might have different security features such as the special dye stuff inside, but if you call and ask they may suggest a way if you can quote the till receipt or something to prove it's not stolen.