When do I get charged for extra postage due to weight?

By default, postage for first-class in a standard envelope starts at $.50 per domestic letter.  An envelope can have up to 5 sheets of 20lb. paper (or 4 sheets of 20lb paper plus a return envelope) before it weighs over an ounce.

For every ounce that is added to a letter, there is an up-charge in postage of $.21.

Any mail that is over 13oz (13.1 and higher) is considered as a parcel by the USPS.