Tier 1 general dependant visa extension

Hi, I am holding ILR through Tier 1 General route(5 years) and applying for my depedants visa extension through post, there previous VISA and extension was through Tier 1 General route, have few questions, answer's appreciated-

1. Is it a good idea to fill in the online form instead of paper form as I am doing this first time. Online form asks few questions for e.g.:- online form doesn't ask for Previous Passport while paper form has.
2. I have filled the form and now need to pay IHS. The application I made is for partner and child. so 2 dependants. Application asks for only 1 IHS number. Will this be same for 2 dependants i.e should I pay 1200 and provide IHS number.
3. How to do payment if the application is made online currently application made by post is 1806.00 GBP so for both its 3612.00 GBP. After paying IHS will it ask for application fees or should it be cheque or bankers draft.

I know these questions are very basic but helpful.
Thanks,