UPS Mailboxes are a good - if expensive - way of getting a PR card couriered to you to your home country.
The reason for being expensive is that you need to rent the box until the PR card is delivered. This could take a long time especially if there is request for updated photos. Also, the rent of the mailbox does not include forwarding charges. These are charged separately, and depend on the final destination of the document.
As others have said before, there is no law couriering the PR cards out of Canada. CIC clearly says that they will not send them outside Canada, but that is their operating procedure. Another way of getting a PR card is to have a friend or relative carry it for you if they are travelling to your destination.
An alternate method is to simply apply for a PRTD if you are outside Canada and have to re-enter without receiving the PR Card.