I'm guilty of overpaying for postage all the time - have you ever done this:
You fill the envelope. Heft it in your hand and decide it's heavy. To be safe you add another stamp or two. Before you're done, your envelope now has $1.17 in postage on it.
Did you just throw your money out the door?
There is an easier way. Go to Office Max/Depot or Staples and buy a postage scale. They're not very expensive, between $10 - $25. Pelouze is one of the largest manufacturers. I have bought a Pelouze digital scale.
Way the envelope on your new scale.
Go to www.usps.gov and click the calculate postage link. Select your format (letter, usually) and enter the zip codes. It will calculate the actual postage for you.
I find it a handy way to use all those old $0.32 and $0.29 stamps I bought years ago and only recently recovered from a "safe place".