I know all of these documents are not required but having them do not hurt.
College does write an invitation letter upon request and i understand it's crucial.
I understand, you must have to fill out i-134.
One affidavit of support is sufficient for father/mother
1. Affidavit of support ( I-134 )
2. university invitation letter (it's crucial if you are inviting them for graduation)
3. letter to consulate (request to grant visa..you can find sample on web)
4. relationship certificate or
5. birth certificate
6. employment verification letter(if you're employed else source of income)
7. h1b visa document/ GC Document
8. paystub
9. tax document(w2)
9. bank letter
10. stock letter (if You have any)
11. university transcript (if you have)
12. your passport, ssn
13. tax return document
You need to fill up DS 160 form(visitor visa if you're applying for visitor visa).
Separately for each candidate(Father and Mother)
I filled up the application myself and emailed them the PDF application form with the confirmation page.
Few days back I did it and they got the visa. :)
They went to Nabil bank(opposite to US Embassy in KTM), submitted the application and confirmation page of application along with fee and they got the interview very next day and got the visa too.
Good Luck.