How to Configure USPS Flat Rate Boxes in the WooCommerce Shipping Plugin

Table of Contents

Understanding USPS Flat Rate Boxes:
Step 1: Confirm the Correct USPS Flat Rate Box and Flat Rate Envelope Size
- USPS Flat Rate Box
- USPS Flat Rate Envelopes
Step 2: Configure the USPS Flat Rate Box and USPS Flat Rate Envelope in the WooCommerce Shipping Plugin
- Example: Configuring a Large Flat Rate Box
Step 3: Understand Why Only USPS Priority Will Return Shipping Rate
If You Want to Display Multiple Shipping Service Options at Checkout
Final Notes

When shipping products from a WooCommerce store based in the United States, there are generally two common approaches for packaging your orders:

Using your own custom shipping boxes
Using USPS Flat Rate Boxes, USPS flat rate envelopes, which are provided free by USPS

Many e-commerce store owners choose to use USPS Flat Rate Boxes because they help reduce packaging costs and provide more predictable shipping rates. This is especially beneficial for heavier items or when shipping to long-distance destinations, where standard weight-based rates may be higher.

However, to ensure USPS Flat Rate rates return properly in the WooCommerce store cart and checkout page, it is important to configure the flat rate box correctly in the WooCommerce shipping plugin settings, such as Shippo Shipping PRO, ShipStation Shipping PRO, ShipEngine Shipping PRO, and EasyPost Shipping PRO.

This guide explains how to properly configure USPS Flat Rate Boxes in the WooCommerce shipping plugins so that the USPS Priority Mail (Flat Rate box) service appears during checkout and returns an accurate live shipping rate.

Understanding USPS Flat Rate Boxes: #

A key detail to remember:

USPS Flat Rate Boxes and Flat Rate Envelopes are classified as Package Types, meaning they determine how a shipment is packed, not the shipping service itself.

This means:

If you configure a USPS Flat Rate Box in your shipping box settings, the plugin will send that package type information via the services API.

When the WooCommerce Shipping plugin selects a USPS Flat Rate box for shipping, the services API will return the corresponding USPS Priority Mail (for Flat Rate box) service, and the shipping rates of USPS Priority Mail will be displayed on the cart and checkout pages.

Other shipping services (e.g., USPS Ground Advantage, Priority Express (non-flat-rate), or other carriers) will not return live shipping rates, because a USPS flat rate box can only be accepted and shipped with USPS Priority Mail.

If you want customers to see and compare different USPS service levels or rates from other carriers (such as UPS or FedEx), then do not configure USPS Flat Rate Box or Flat Rate Envelope package types in the shipping box configuration. Flat Rate packaging only returns USPS Priority Mail and will prevent other services from appearing. Instead, configure a regular package box type, such as parcel, or package, in the settings.

Step 1: Confirm the Correct USPS Flat Rate Box and Flat Rate Envelope Size #

USPS Flat Rate Box #

Each USPS Flat Rate Box has a specific size and type. To obtain accurate live shipping rates, your configured flat rate box dimension must match the exact USPS specifications on the USPS website.

Flat Rate Box Type	Dimensions (inches)	Notes
Small Flat Rate Box	8.69″ × 5.44″ × 1.75″	Good for small, lightweight items
Medium Flat Rate Box 1 (Top Loading)	11.25″ × 8.75″ × 6″	Larger than small, cube-like
Medium Flat Rate Box 2 (Side Loading)	14″ × 12″ × 3.5″	Same pricing as Medium Flat Rate Box 1
Large Flat Rate Box	12.25″ × 12″ × 6″	Best for larger or multiple items
APO/FPO Large Box	12.25″ × 12″ × 6″	Discounted for military shipments

Important:

The dimensions you enter in the WooCommerce shipping plugins must match exactly. If the dimensions differ (even slightly), the services API will not return the USPS Priority shipping rate.

USPS Flat Rate Envelopes #

USPS Flat Rate Envelopes are ideal for documents and small items. They are also shipped under USPS Priority Mail and follow the same packaging rules as flat-rate boxes.

Flat Rate Envelope Type	Dimensions (inches)	Notes
Flat Rate Envelope (Standard)	12.5″ × 9.5″	Document-sized
Legal Flat Rate Envelope	15″ × 9.5″	Longer legal-size documents
Padded Flat Rate Envelope	12.5″ × 9.5″	Slightly thicker padded envelope
Small Flat Rate Envelope	10″ × 7″	Often used for compact/light items

Envelope Height:

For the USPS Flat Rate Envelope height, you may assign a practical height (thickness) based on how full the envelope typically is when packed. It does not need to match the exact flat thickness, but it should reasonably represent the actual shipping scenario.

Do NOT change envelope dimensions to “fit products better.” USPS will only return flat rate pricing when the exact USPS envelope type is selected and its official size is used.

Step 2: Configure the USPS Flat Rate Box and USPS Flat Rate Envelope in the WooCommerce Shipping Plugin #

Go to your WooCommerce Shipping plugin (such as Shippo Shipping PRO, ShipStation Shipping PRO, EasyPost Shipping PRO, and ShipEngine Shipping PRO)
Go to the “Parcel Packing” Tab
Click “Add Box“
Input the exact flat rate box’s (or flat rate envelope) dimensions, and the empty box/envelope weight
Select Box Type: Choose the appropriate packaging type, for example, USPS Large Flat Rate Box, or the specific Flat Rate box or envelope you intend to use.
Set the “Max Items” and “Max Weight” values for the configured shipping box or envelope (if applicable). This ensures the plugin does not attempt to pack more products than the box or envelope can realistically hold.
Click “Save changes” to save the setting.

Example: Configuring a Large Flat Rate Box #

Box type: USPS Large Flat Rate Box

Dimension: 12.25 x 12 x 6″

Weight (Empty Box weight): approximately 0.8 lbs (Please include packing material weight if applicable)

Max items: 5 (This prevents the plugin from packing more than 5 products into this box)

Max weight: 5 lbs (This limits the total product weight for this box to 5 lbs, ensuring the plugin does not overfill it)

Weight:

Enter the empty box weight (with no products inside). If you use packing materials such as tissue, bubble wrap, or kraft paper, include their approximate weight as well. This ensures the plugin calculates the final package weight more accurately and avoids under-charging for shipping.

Padding:

If you use padding or cushioning inside the box, enter an approximate padding value. This will reduce the box’s internal usable dimensions so the packing algorithm accounts for the space taken up by the packing material.

Once the Flat Rate Box is configured, if the plugin selects a USPS Flat Rate Box for packing, the API will return the corresponding USPS Priority Mail (Flat Rate) shipping rate. This rate will be displayed in the cart and during checkout.

If the plugin selects a regular (non–flat rate) box, then applicable USPS shipping services (and any other enabled carriers’ shipping services) will return their live shipping rates and display them in the cart and checkout page.

Step 3: Understand Why Only USPS Priority Will Return Shipping Rate #

USPS Flat Rate Boxes and Envelopes are designed specifically for USPS Priority Mail. Because of this, they cannot be rated or shipped using other USPS services (such as Ground Advantage or Express), nor can they be used with carriers like UPS, FedEx, or DHL.

So during the cart and checkout:

Other shipping services will not appear when the WooCommerce Shipping plugin selects a USPS Flat Rate box or envelope for shipping an order.
Only USPS Priority will return a shipping rate.

This behaviour is correct and required by USPS rules.

If You Want to Display Multiple Shipping Service Options at Checkout #

If you want to display and compare different USPS service levels and also show live shipping rates from other carriers such as UPS, DHL, or FedEx:

✅ Use a general shipping box type, such as Parcel or Package, instead.
❌ Do not set up a USPS Flat Rate Box or a USPS Flat Rate Envelope

In this case, the WooCommerce shipping plugin will display:

applicable USPS shipping services rates (not flat rate), along with
Any other available shipping services or carriers you have enabled

Final Notes #

Ensure your product dimensions and weights are accurate, so the WooCommerce Shipping Plugin’s packing algorithm can correctly determine which shipping box or envelope to use.

You can enable debug mode to test the WooCommerce shipping plugin and observe how the plugin selects shipping boxes during checkout, then adjust your configuration if necessary.

Please note that the plugin may not always choose the shipping box or envelope you expected because it selects the shipping box or envelope based on product size, volume, and weight. If this occurs, we recommend adjusting settings such as Max Items, Max Weight, or Padding, then test again to achieve the preferred packing result.

Always verify that the Flat Rate Box or Flat Rate Envelope dimensions you enter in the plugin exactly match the dimensions provided on the official USPS website.