You can pay it now online: http://www.cic.gc.ca/english/inFORMation/fees/index.asp
It will allow you to save your receipt (and it will email you a pdf copy as a backup). Then you can upload that to the CIC via Case Specific Enquiry: https://secure.cic.gc.ca/enquiries-renseignements/canada-case-cas-eng.aspx. You can also mail it in to the CIC CPC-Mississauga with a covering letter containing your details (name, address, phone, email, application number, UCI).