Shipping guide
How to ship letters, flats, and packages
Choose the mail type that matches the actual size, thickness, shape, and flexibility of your item. Accurate measurements help prevent delays and postage adjustments.
Letters
- Minimum: 3.5 inches high × 5 inches long × 0.007 inch thick.
- Maximum: 6-1/8 inches high × 11-1/2 inches long × 1/4 inch thick.
- Must be rectangular.
- Thick, rigid, lumpy, or uneven envelopes may not qualify as standard letters.
Example: a greeting card inside a standard flexible envelope is usually a Letter.
Flats
- A Flat is larger than letter size in at least one dimension.
- Maximum: 12 inches high × 15 inches long × 3/4 inch thick.
- It should remain flexible, rectangular, flat, and evenly thick.
Example: a flexible large photo or document mailer under 3/4 inch thick may qualify as a Flat.
Packages
Choose Package for boxes, rigid mailers, thick or lumpy items, merchandise, and anything that does not meet Letter or Flat rules. Enter the packed length, width, height, and weight.
USPS generally limits Ground Advantage packages to 130 inches in combined length plus girth. Lower limits or added charges can apply to other services and unusually large packages.
Eligible TrackALetter letters and flats include barcode-based machine-scan tracking. This is not the same as Certified Mail, signature confirmation, or guaranteed package tracking. Scan availability can vary by mailpiece, route, and USPS equipment events.
Common mistakes
- Guessing weight instead of using a scale.
- Measuring the item before final packaging.
- Using Flat for a rigid or uneven mailer.
- Leaving out package dimensions.
- Choosing Letter for an envelope thicker than 1/4 inch.
Prohibited and restricted items
Do not mail prohibited, hazardous, illegal, or restricted contents. Review the official USPS shipping restrictions and the TrackALetter Acceptable Use Policy before creating a label.