The university will not write welcome letter for sure as that is for a different intent. However, your International office should give you some letter stating that you are graduating and inviting them for your graduation. I invited my parents twice for my graduation. I don't know what they did in their end ( the usual take appointment stuff) but I sent a letter from my school stating that I am graduating and inviting them for that event. Also, you don't have to mail them the letter, you can send them the scanned copy if you already haven't done so .