I think you will hit your road block when you attempt to send them; i.e. my understanding is that both UPS and Fed Ex require a street address or they simply will not accept the package.
All you really need is "proof of mailing," not "proof of receipt." You just get the stamped receipt from the P.O. for each SAR that you mail and attach it to the file copy. It's easier and less expensive.
To eliminate the whole mess, consider e filing.