Postcard:

• Must mail first class to get the postcard rate. (If the organization is nonprofit, nonprofit is less expensive than postcard rates but the delivery period of nonprofit is slower)
• Minimum size: 3 1/2 x 5
• Maximum size: 4 1/4 x 6
• Minimum thickness: .007
• Maximum thickness: .016
  

Letter:

• Minimum size: 3 1/2 x 5
• Maximum size: 6 1/8 x 11 ½
• Minimum thickness: .009
• Maximum thickness: .25

Flat:

• Minimum size: 6 1/8 x 11 ½
• Maximum size: 12 x 15
• Minimum thickness: .25
• Maximum thickness: .75

Machinable Parcel:

• Parcels that can be processed on Postal Service Equipment
• Minimum size: 6 L x 3 H x .25 W
• Maximum size: 34 L x 17 H x 17 W
• Minimum weight: 6oz
• Maximum weight: 35lbs

Irregular Parcel:

Parcels that do not meet the dimensional criteria of machinable parcels including: rolls & tubes up to 26" long & merchandise samples not individually addressed; articles that are not letter or flat size.