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 and greeting cards
- Shape
- Rectangular and flexible
- Tracking
- Tracking updates appear when USPS processing data becomes available.
Flat
- Best for
- Large flexible envelopes, documents, and photos
- Shape
- Large, flat, and flexible
- Tracking
- Tracking updates appear when USPS processing data becomes available.
Package
- Best for
- Boxes, rigid mailers, and thick items
- Shape
- Rigid or thicker items
- Tracking
- USPS package 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
Tracking updates appear when USPS processing data becomes available.
Letter and flat updates come from USPS mail-processing events and aren't guaranteed. An acceptance event may not appear, and updates can be delayed or incomplete. A delivery indication is not necessarily a physical mailbox scan.
Which service should I choose?
- Letter
- Service: First-Class Mail
- Packaging: Flexible envelope
- Tracking: Tracking updates appear when USPS processing data becomes available.
- Flat
- Service: First-Class Mail
- Packaging: Flexible large envelope
- Tracking: Tracking updates appear when USPS processing data becomes available.
- Package
- Service: Ground Advantage
- Packaging: Own package or eligible soft pack
- Tracking: USPS package tracking
- Priority / Express
- Service: Coming soon
- Packaging: Future eligible own or USPS packaging
- Tracking: Coming soon
Mailing a standard envelope? Read the First-Class Letter postage guide.
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.