My mother in law got visa 3 weeks before. In addition to above documents, you can also send you certificate of enrollment (something to show how many semesters you have attended). You department can help you with that..It proves you are studying here now..And if you intend to cover their cost in US and will mention that in invitation letter, its better to show some bank balance in US too.. I showed around 5000. Also , on your parents side they may need to show proof of their work/ bank account/lalpurja etc. Also, send a copy of your I94 and visa page . You can actually print it online.

But if you are inviting them for graduation, i think things are simples. In my case, i just invited her for visit.