Not true. These are not PO Boxes. With the UPS Store and other private mailbox providers you get a street address not a PO Box. I know for a fact that many companies use such addresses as their primary correspondence address with the CRA, and even with their bank. Even myself I have used a mailbox address from the UPS Store in Guelph for years before I rented my own office, and I have received all my CRA mails there.
Generally a PO Box from Canada Post would not work but there is a work around in some cases.