When you submit your application, you have to pay fees for application processing + RPRF, however they give you option to pay later for RPRF.
This means you have chosen to pay later, so that's why they are asking you to pay this fees.
I also did same, and now at 7th month they asked me few documents and asked me to pay for RPRF. I have paid 980 Dollars for myself and my spouse.
Have they given you any link to follow and pay? I was given a link to follow and pay but that link was taking me nowhere, after all attempt since there is no channel to contact CIC via email to get advise to what to do in this situation, I searched for RPRF payment, google took me to CIC website payment page, and I paid my fees over there and got receipt of payment, same I have uploaded.
You need to do same, I have uploaded it yesterday, if you have time wait for me, if there will be any problem with it CIC will notify me and I can update you.