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USPS Support and FAQs

Updated

Overview

This article provides guidance on USPS service behavior, shipment options, tracking details, refunds, zones, and other commonly encountered USPS scenarios when shipping through EasyPost.

Prerequisites

USPS Zones and Shipment Date Behavior

How USPS Zones Work

USPS calculates pricing for certain mail classes based on weight and distance using Postal Zones. Zones are determined using the origin and destination ZIP codes. 

  • Zone 1 represents the shortest distance
  • Zone 9 represents the farthest distance

A zone can be identified using the USPS rate charts or EasyPost's Zone Lookup Tool. 

Advancing Ship Dates

USPS labels may be generated with ship dates up to 6 days in advance.

Ship Date Acceptance

USPS requires packages to be accepted on the printed ship date. 

In practice, USPS generally accepts packages a few days before or after the printed date, provided postage rates have not changed during that time. 

EasyPost cannot guarantee acceptance outside of the official USPS window. 

USPS Mail Types, Size Limits, and Packaging

USPS First-Class Parcel Restrictions

First-Class parcels must:

  • Weigh less than 16 ounces
  • Not exceed 22" x 18" x 15"

First-Class Mail Large Envelopes (flats)

Flats must:

  • Be rectangular
  • Measure no more than 15" x 12" x 3/4"
  • Weigh 13 oz or less

Items exceeding these limits are classified as packages.

To designate a flat in EasyPost:

"options": {
    "predefined_package": "FlatRateEnvelope",
    "weight": 1,
}

Soft Pack or Padded Envelope Cubic Pricing

Soft pack and Padded Envelope Cubic Pricing are supported for USPS Priority Mail and Ground Advantage. See the USPS Cubic Pricing Guide for more information. 

Refer to the Predefined Packages section of the EasyPost API Docs for a complete list of options.

Signature Options and Restricted Deliveries

Adult Signature vs. Signature Confirmation

USPS provides multiple delivery confirmation types.

Option Description
SIGNATURE Requires a signature from someone at the delivery address.
ADULT_SIGNATURE Requires the signature of an adult (21+ with ID) at the delivery address.
SIGNATURE_RESTRICTED Requires the signature of the specified addressee, or authorized agent.
ADULT_SIGNATURE_RESTRICTED Requires the signature of the specific addressee, or authorized agent, who is 21 years of age or older.
NO_SIGNATURE Leaves the package at the door with no signature required.

Example

"options": {
    "delivery_confirmation": "ADULT_SIGNATURE"
}

Shipping Restricted Items

Tobacco Shipments

Mailing tobacco is heavily restricted. USPS allows mailing only under specific conditions (e.g., cigars, intra-state shipments, regulatory purposes, returns).

When shipping tobacco, use:

"options": {
    "delivery_confirmation": "ADULT_SIGNATURE_RESTRICTED"
}

A USPS employee must verify age and accept the package in person.

Tracking Behavior and Delivery Expectations

Tracking First-Class Letters and Flats

USPS now provides tracking updates for First-Class letters and flats via Intelligent Mail Barcodes (IMb). Notes:

  • USPS.com cannot display these tracking numbers.
  • Estimated delivery dates may not always populate.
  • Tracking is available for commercial barcoded mail through EasyPost.

Predicting Delivery Dates

EasyPost does not provide pre-purchase delivery predictions. USPS provides a Service Commitment lookup for Priority Mail and Priority Mail Express.

Only Priority Mail Express offers guaranteed delivery.

Priority Mail Express Delivery Dates

USPS does not return delivery_days or delivery_date_guaranteed by default. EasyPost can enable this feature upon request.

Example of guaranteed date in the rate response:

"delivery_days": 1,
"delivery_date_guaranteed": true

TrackingDetail Status

Possible values of the TrackingDetail status include:

  • "unknown"
  • "pre_transit"
  • "in_transit"
  • "out_for_delivery"
  • "delivered"
  • "available_for_pickup"
  • "return_to_sender"
  • "failure"
  • "cancelled"
  • "error"

Example

{
  "object": "TrackingDetail",
  "message": "Pre-Shipment Info Sent to USPS",
  "description": "",
  "status": "pre_transit",
  "status_detail": "status_update",
  "datetime": "2025-04-09T20:40:17Z",
  "source": "USPS",
  "carrier_code": "",
  "tracking_location": {
    "object": "TrackingLocation",
    "city": null,
    "state": null,
    "country": null,
    "zip": null
  },
  "est_delivery_date": null
}

Undelivered, Misdelivered, and Returned Shipments

USPS Package Intercept

A USPS label cannot be edited once created. If the address is incorrect, USPS may redirect the package through Package Intercept®. 

Package Intercept:

  • Must be requested directly from USPS
  • Requires a trackable barcode
  • Is not currently supported through EasyPost

Tracking Returned-to-Sender Packages

If USPS marks a package Delivered and later returns it:

  • The tracking status remains 
  • USPS does not issue return-specific tracking updates
  • EasyPost stops receiving new events

To distinguish scenarios, use:

Refunds and Special USPS Rates

Priority Mail Express Funds

If USPS misses a guaranteed delivery window, contact EasyPost Support for assistance with the request. This process differs from unused-label refunds. 

Refunds for Unused USPS Postage

Refunds can be requested if:

  • The label is unused
  • The label is in pre_transit
  • The request is made within 30 days

Refunds may take USPS 15-30 days to process. After USPS approval, EasyPost refunds the EasyPost label fee automatically. 

API and Dashboard refund options are available. 

Refunds for First-Class Letters and Flats

These labels fall outside USPS's standard refund process and cannot be refunded through EasyPost.

Media Mail and Library Mail Eligibility

EasyPost offers support for both USPS Media Mail and USPS Library Mail rates. In order to receive Media Mail or Library Mail rates, set the Shipment Option special_rates_eligibility value as shown below. 

"options": {
    "special_rates_eligibilty": "USPS.MEDIAMAIL"
}

or:

"options": {
    "special_rates_eligibilty": "USPS.LIBRARYMAIL"
}

FAQs

Q: Can USPS provide overnight delivery?

Yes. Priority Mail Express provides overnight or 1-2 day delivery to most U.S. addresses.

Q: Can USPS packages be shipped earlier or later than the printed ship date? 

USPS expects acceptance on the printed date, but usually accepts packages a few days before or after if postage rates are unchanged.

Q: Is there tracking for returned-to-sender USPS packages?

No. Once a package is scanned "Delivered," USPS does not issue additional tracking events for returns.

Q: Can USPS First-Class letters or flats be tracked?

Yes, when purchased through EasyPost. Tracking appears through IMb events, not on USPS.com.

Q: Can USPS postage be refunded after a package is returned to sender?

No. Once accepted into the USPS mail stream, postage cannot be refunded.

Q: Can Media Mail or Library Mail be requested through EasyPost?

Yes. Use the special_rates_eligibility Shipment option.

Q: Can the ship date be advanced?

Yes. Up to six days in advance.

Additional Resources

API Docs - Shipment Options

API Docs - Trackers

Tracking Guide

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