Visitor, Thanks for your response though late.

By the way after application of I-539, if they return, what they need to submit at Airport instead of I-94, which gives proof that they left. receipt Notice??

So summary is:

1. For extension of visa for couple- the filing fee is only $300.

2. No additional documents required except copies of passport, visa and I-94.

3. Can be filed Online and send hard copies by mail.

4. With receipt notice, there is no problem at London though 1st visit visa time (1st 6 months stay) has expired.