All mail too large to fit in an envelope should be sent in a USPS flat rate box.

UNITED STATES POSTAL SERVICE (USPS) REGULATIONS
The USPS prohibits the mailing of any of the following:
Potentially hazardous materials that are not properly marked and packed;
Perishable items that are not properly marked and packaged;
Correspondence containing any vile, or obscene material and matter inciting violence or terrorism;
Solicitations that mimic billing statements, unless accompanied by a prominent disclaimer; and
Correspondence that bears deliberate imitations of postal markings and/or postal trademarks (e.g. “Priority Mail,” etc.).
Correspondents are personally responsible for the content of each item of correspondence they send through the USPS.
Any violation of laws governing correspondence will be referred to postal authorities and to appropriate criminal authorities. The sender may be subject to civil or criminal penalties and/or federal prosecution for violation of postal laws.