Mail of any kind for transport by the United States Postal Service should be enclosed in an appropriate envelope or parcel and sealed. The types of enclosures determine the mailing classification.
In general, any material which causes a bulge should not be mailed in an envelope. Such materials will not only jam or damage the mailing machines, but can also cause serious injury to Mail Services employees.
Enclosures, designed so that the address appears in a window envelope, must not be stapled to prevent slippage of the address from window view. If the address enclosure does not properly fit the window, use an envelope without a window. Please do not staple mail into envelopes.