Using USPS flat rate envelopes/boxes + click-n-ship is a no brainer to me!!! You get free delivery confirmation as previously mentioned AND a free online address book so when you ship to previous customers, you just click to submit their address. The great thing is you can access that address book from anywhere you are!!! Rates are great and in 4+ years, I have not had a lost package!!!
Makes it easy for accounting also as you can print out your postage use for the month as well...
Jim