What documents should I submit after receiving the ITA for Canada Express Entry Success?

Now a new option would have opened in your profile. After you fill out these details and reach the next step, there will be separate sections to submit all your documents, so you will not miss out any part. Following is the link which details out all the documents required:

Applications received on or after January 1, 2015, for permanent residence programs subject to the Express Entry completeness check

In the section where you upload the documents, there would be an additional section of Letter of Explanation (LOE). If any section(s) require explanation, then write it in a single letter and submit it there.

Some points to note while submitting documents:

1. For a single section, eg: Proof of funds, you have to submit a single file only, size of which should not exceed 5 MB.
2. In educational documents section, please do not forget to add IELTS certificate and degree evaluation certificate. Though it is not called out in their details, but it is required. I have seen applicants getting request to submit it later.
3. For the police clearance certificate, attach certificates of all the countries you have lived in for more than 6 months. Each PCC should cover your entire stay in that country. A lot of applications are being rejected straight away nowadays, as people miss PCC for a single state or for a little time duration.
4. LOE is an important section. Please use it judiciously and explain yourself fully.
5. Answer all the statuary questions and other details truthfully or risk bring blacklisted. Do not leave out any detail.

Regarding payment, 550 CAD are required upfront. Additional 490 CAD have option to be paid along with 550 CAD or can be paid later. But would advise to pay whole amount of 1040 CAD during summiting your application, otherwise it will waste time later on when they get back to you for the pending amount.

Regarding GCMS notes, they are only to check progress on your application. Ask for them later on if you do not see any movement in your application for more than 3 months. Please note that you do not get email when your background check, etc starts. So you would have to login to your application regularly to check for updates. If you have a relative or friend who is permanent resident or citizen in Canada, they can get it on behalf of you and would cost just 5 CAD.