There are different stages to the application. Once the necessary checks have been made and the application looks complete and fit for the next stage, you will get a request in your account to pay the RPRF fee. This is usually a good sign.
What is the approximate time that they will ask for RPRF .... I passed my medical on september 18 ??
However, if you really want to pay the fee before they ask for it then do the below.
https://eservices.cic.gc.ca/epay/welcome.do?lang=en
To pay your RPRF, please follow the below instructions:
Go to our Online Payment webpage
Read the instructions on each page, and click “Continue” to proceed
Select the “Immigration” link, followed by “Right of Permanent Residence Fee”
Enter the number of fee payments (1 or 2) in the box beside the fee of $490
Click “Submit”
Proceed to pay your fees by selecting “Pay” and then follow the instructions to pay with Visa, American Express or Mastercard.
Make sure you save a copy of your receipt. You will need it to raise a case specific enquiry here.
https://secure.cic.gc.ca/enquiries-renseignements/canada-case-cas-eng.aspx. That will be attached to your case.
It might be best to wait till they request for it. Good luck