Product flat table already contains special price column, you don't need to join this.
Coder And Traveller
Yeah, the doc is missing I will update the docs, I just raised an issue,
Please use a single thread for better communication.
You need to check the mentioned payment gateway docs for this.
Take a reference of this file,
If you are customizing the theme, then you have to customize the copy of this file also. So that your classes get compatible with your current theme.