@BlueMatia, There is no way to submit documents to CRA when you NetFile (or Efile) your tax return. Only if you choose to "paper file" or mail in your tax return are you able to submit any documents. But as a general rule, you will only need to submit these documents if CRA requests that you do so.
You may be asked to submit proof of a deduction or claim if CRA chooses to do a "Pre-Assessment" on your tax return. If this is the case, you will need to submit the documents prior to your tax return being assessed and your refund is issued. Failure to submit the documents will result in your tax return being assessed without the deduction or claim being reviewed.
If you are asked to submit documents "after" your tax return is assessed, this is called a "Post Assessment Review". If you do not submit the documents to the tax centre which has requested them, your claim or deduction will be disallowed, and your tax return will be reassessed.
In both cases, it's important that you provide the reference number on the letter you have received from CRA and deal with that particular tax centre. If you have questions or require more time to gather your documents, you should contact them directly at the number provided.
Here's a link that explains how to submit documents online to CRA: How to submit documents to the CRA.
