the cashier may be smarter than you think. I used to switch price tags all the time.
When i would switch tags on clothing i would go to a male cahsier because i thot he wouldnt notice.
but the last time i did it the cashier was like hmm this isnt right so i freaked out and said never mind i dont want it lol . but the cashier you had may have rang that item up before , or if you tried to put a $5 tag on a $30 dollar dress they could tell the difference.