University will send one letter and you can send your personal invitation letter as well. And, if you are a TA/GA student, you could send a bank balance showing your a little  support because most of the support should come from your parents as you are in a student svisa so it's better to show bank balance with your parent's income. I heard that these days they give visa for less than 5 years. I met one of my friend's parents they were given 6 months visa.