Can you use a forever stamp on something that weighs more than an ounce?

Use the postage calculator on the USPS website to figure out the first class postage due for the weight of your envelope, then divide by the current first class stamp price, then round up to determine how many forever stamps to use.

As of 4/2009, forever stamps are worth $0.42 each, but their value will increase to $0.44 next month when postage goes up.