There was a discussion on this already and multiple opinions as usual. Search that AOR thread if you want to read those messages.
In considered opinion, it is apparently a legacy message from the old system. Under EE, the payment is made online and confirmation arrives in your MyCIC account so there should be no requirement to print out and send a copy of the receipt (which CIC sent you in the first place!) back to CIC to confirm that you have received their confirmation about your payment!
There was a discussion on this already and multiple opinions as usual. Search that AOR thread if you want to read those messages.
In considered opinion, it is apparently a legacy message from the old system. Under EE, the payment is made online and confirmation arrives in your MyCIC account so there should be no requirement to print out and send a copy of the receipt (which CIC sent you in the first place!) back to CIC to confirm that you have received their confirmation about your payment!