A few ways to make sure you cut the costs down are to do the following:
1) do not EVER bring to USPS and have them calculate the shipping for you. That will cost you about $1 each package for the convenience of them doing it. I personally use PayPal shipping, but there are others out there. Find the one you like and enjoy the savings!
2) on theBench, MOST of us are perfectly fine with and encourage reusing shipping supplies. You can reuse the bubble mailer and that will save 50-75 cents each time as well.
3) low end cards, use a non machinable stamp to ship. Those are 75 cents each. They can ship up to two ounces on a Non Machinable stamp, so, use that extra weight allowance to protect the card. I use thin cardboard on both sides of the card. Usually I cut up a cereal box, pasta box or whatever I have about to go out to the recycling. Then, on the leading edge of the envelope, I put a strip of tape front and back. That helps prevent being caught on the machine IF the USPS decides to send the non machinable envelope through the machine. I have shipped this way 100s of times and not once has a card been lost or damaged.
Hope all this helps you to save money on shipping.