Trying to be entirely paperless? Perfect. Me, too. Far less impact on the environment/unparalleled speed of delivery.
But there are times when USPS is essential. Handwritten notes are just one very much appreciated example.
In which case, you're going to need a stamp.
My top pick, either as a gift for someone else or to use yourself: Zazzle personalized postage. I've tried others but continue to choose this company. I like the print quality, color saturation, quick turnaround time and variety of stamp sizes. And I typically order the largest version that they offer simply because it's a beautiful addition to the envelope. Not for bills but for someone who would appreciate the extra effort. Great birth/or other event announcement. And a cinch to do just by uploading your pictures.
ADDITIONAL CONCEPT: If you live in the SF Bay Area, you're no doubt familiar with FLAX, an extraordinary art supply store. Their stock of paper products is pretty unparalleled/let me know if you have a better resource. I was shopping for A-7 (5.25" X 7.25") envelopes and cardstock to mount my own pictures.
Then I found translucent envelopes. Very cool to use for mailing something special, like a photograph or your own photo stationery. FLAX's price was substantially higher than other online sources I subsequently found. So, if interested, Google "translucent envelopes" for a good deal. I ordered some today from envelopemall.com. You may find a better source/value. Let me know.