Starting with last tax season, I began using priority mail using my Pitney Bowes account.  I get my pretty little Pitney Bowes label printed out, stick it to one of my own envelopes and let it fly.   I get my notifications when it is delivered and life is good.  The only time I had an issue was last year.  I sent a tax return out to one of my clients that winter in Arizona and doesn't return until the first part of May.  Well last year she decided to depart at the end of March.  She left Arizona one morning and  I got notification that it was delivered into her mailbox that afternoon.  Eventually she got things squared away.  But other than that, everything I have sent out the last couple of years has been delivered timely as promised.