Shipping guide
Letter vs flat vs package: USPS mailing guide
Choose the mail type that matches the size, thickness, shape, and flexibility of your item before creating a label.
Quick comparison
Pick the mailpiece that matches the item
Letter
- Best for
- Flexible envelopes, greeting cards
- Shape
- Rectangular and flexible
- Tracking
- Machinable scan-based updates to delivery when available
Flat
- Best for
- Large flexible envelopes, documents, photos
- Shape
- Large, flat, flexible
- Tracking
- Machinable scan-based updates to delivery when available
Package
- Best for
- Boxes, rigid mailers, thick/lumpy items
- Shape
- Rigid or thicker items
- Tracking
- Carrier tracking
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, flexible, and evenly thick.
- 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.
Tracking expectations
Eligible TrackALetter letters and flats include scan-based barcode events. This is not the same as Certified Mail, signature confirmation, or guaranteed legal proof. Scan availability can vary by mailpiece, route, and USPS equipment events.
Before you buy
Common mistakes
- Guessing weight instead of using a scale.
- Measuring 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 USPS shipping restrictions and TrackALetter Acceptable Use before creating a label.
See the official USPS shipping restrictions and TrackALetter Acceptable Use Policy.
Ready to choose the right label?
Confirm the mail type, weight, dimensions, and full total before purchase.