I'm going to add that even though some sororities may say it is their policy to let NMs wear letters, campus tradition might dictate otherwise. Also the TYPE of letters that may be worn (sewn on, screen print etc) may be something that factors in from campus to campus. (I'm guessing, mom, you're like me and remember the days where if we got a screen print from the bookstore we were happy.
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Just to be safe, stick with pens, notecards, totebags, stuffed mascots etc. for right now. The last thing you want is for your daughter to get some sort of shirt that's inappropriate.