B2 Visa extn - confused about supporting docs

[celticsfan]

I'm planning to file for an extension (B2 visa) for my parents but am quite confused about the supporting documents I need to file.

On the USCIS site the only required documents mentioned in the I-539 instructions booklet are:
1. I-539 form + $300 fee
2. Copy of I-94
3. written statement explaining reasons for request

On Immihelp, the foll. documents are listed as required in addition to the above:
a. copy of passports
b. proof of financial support
c. copy of air tickets
Are these additional documents really needed?

For proof of financial support, is a letter from my bank sufficient or do I need to file I-134 with all its supporting docs (bank stmt, employer stmt, paystubs, w2, letters etc)?

Thanks in advance for any advice.
~cf

[immihelp]

1. None of those documents are mandatory. It is up to you what you want to send to convince USCIS to approve the extension. We have written what we have found to be useful in getting the job done.

2. https://www.immihelp.com/usa-visitor...sor-documents/