I would take the liner back there and dump it in the middle of the showroom floor and tell them to find the tag. Bring witnesses with you to if you can. Then remember to talk loudly, that always helps, reminding them that "_______" their competitor also sells liners.
You should do this when the store has some other customers in there also, whenever you get left alone, tell one of the customers your story. I am sure that things will happen quickly in your favor.