Yes, you must submit the tax form when asked. But it's very easy, just go to Tax Center in Management Area.

There are different forms for US and non US citizens and companies, choose your form and follow instructions.

You can submit your form entirely online, and - in case you aren't a US citizen - you DON'T need an ITIN anyway: just leave that field blank.

- I can submit the tax forms before I demand payment?

Yes, you should submit form before demanding payment

- If I want submit later, now able upload files to pending?

I already submitted form so I don't know

- How about "Security measure for Tax Forms: please upload a copy of your ID with photo (such as identity
card, passport, driver's licence, etc). Note: this is NOT a photo of yourself."?
I take this via phone(low quality), no scan is okay?

I guess if it is readable it's ok

- When I try to submit, appear warning box, after submit tax form I can't edit again?

I can't edit my submitted form, just view it without changes.