Customers push carts while shopping at a Costco Wholesale store in Hackensack, New Jersey.

Don't you know you're not supposed to squeeze the Charmin toilet paper — or charge sales tax on it?

A New Jersey couple on Friday filed a class-action lawsuit against Costco Wholesale and several of its stores, claiming the membership-only warehouse giant has been illegally overcharging them — and potentially hundreds of thousands of other customers — by charging sales tax on toilet tissue purchases in violation of state law.