It's pretty much OK to ask the store clerk anything that isn't vulgar, obscene, or indecent (swear words or talking about/mentioning sex or bodily fluids unnecessarily). Yeah, there's no problem with "TP". Heck, there's a bit of an aversion toward saying "toilet" in America. (Ever noticed that the word "bathroom" is always used for that here? Some Britons like to make fun of us for that.) "TP" raises that discomfort a little less at least for some.
Other than a number of very specific "bad words", which I'm sure you're aware of, it's not word choice that affects politeness in English, rather it is (literally) tone. Other than just not saying the aforementioned swear words and not raising your voice, I don't think non-native speakers need to worry about it.