A shop at my local railway station is selling:
TOILET PAPERS
Paper is an example of a noun which can be used uncountably to mean the substance itself or countably to mean one item made out of it. But a paper/papers generally refers to newspapers or academic, legal, government or identity documents, or something like them. Everything is just paper. I can give you a newspaper or two, but I can’t give you a toilet paper or two – I can only give you two sheets or two rolls.
Toilet papers, if they are anything, are newspapers or academic etc documents which can be read while sitting on the toilet, which is basically all of them anyway. Indeed, the toilet may be the best place for some of them.
Never Pure and Rarely Simple 