I'm going to ask my roommate to stop buying single-ply toilet paper
Is there a reason that you dislike single-ply?
Yes, I hate the cardboardy toilet paper. We need to go for two-ply
You are expected to purchase the products that you like for yourself. Your roommate is not responsible for your preferences and he/she might be trying to save money. Asking her to buy what you like will make you appear to be inconsiderate.
Ok, I'll just tell her she doesn't know how to live a good life and maybe she'll decide to buy the good toilet paper.
Do you think that will convince her to buy different toilet paper? Maybe if you buy the toilet paper that you like and stock it in the bathroom, she will eventually decide that she likes it better also and start buying it too.