I am instructed to send my Passport , Photos ,Annex A along with a self-addressed shipping label along with its commercial invoice and a returning envelope from FedEX/UPS to CPCO-Ottawa.But the mailing address is a PO BOX No. UPS and FedEx are saying only USPS does deliver to PO Box.Now my question is: Can I buy FedEX envelope and shipping label from online? Can I put them along with my other documents (photo,Passport, Annex A) on a USPS priority mail?Is there any issue with that?