Complete Guide to USPS Flat Rate Boxes: Sizes, Costs, and Tips

Virginia Miller
April 8, 2025

Frequently Asked Questions

What is USPS Flat Rate shipping and how does it differ from regular Priority Mail?

USPS Flat Rate shipping is a service that charges a fixed price regardless of package weight (up to 70 pounds) or destination within the US. Unlike regular Priority Mail, which calculates costs based on weight, dimensions, and shipping zones, Flat Rate shipping requires using USPS-provided packaging but offers predictable pricing. Both services deliver in 1-3 business days and include tracking and up to $50 in insurance, but Flat Rate is generally more economical for heavier items traveling longer distances.

How much do USPS Flat Rate boxes cost in 2024?

In 2024, USPS Flat Rate box prices are: Small Flat Rate Box ($10.40), Medium Flat Rate Box ($18.40), and Large Flat Rate Box ($24.75). Flat Rate Envelopes cost $9.85, Legal Flat Rate Envelopes are $10.15, and Padded Flat Rate Envelopes are $10.60. These prices apply to domestic shipments and are subject to change. Commercial pricing discounts are available for high-volume shippers through platforms like Shipping APIs, Stamps.com, or USPS Click-N-Ship.

When should I use USPS Flat Rate vs. regular Priority Mail for my eCommerce shipments?

Use USPS Flat Rate when shipping heavier items (over 2 pounds) to distant locations (zones 5-9), when you need predictable shipping costs for budgeting, or when sending dense items that would be expensive with weight-based pricing. Choose regular Priority Mail for lightweight packages (under 2 pounds), items shipping short distances (zones 1-4), oddly-shaped products that don't fit efficiently in Flat Rate boxes, or when your own packaging is more suitable. Analyzing your typical shipping patterns can help determine which option saves more money for your specific business needs.

What are the weight and size limits for USPS Flat Rate boxes?

All domestic USPS Flat Rate boxes have a maximum weight limit of 70 pounds. Size limits vary by box type: Small Flat Rate Box (8⅝" × 5⅜" × 1¾"), Medium Flat Rate Box (11¼" × 8¾" × 6" or 13⅝" × 11⅞" × 3⅜"), and Large Flat Rate Box (12¼" × 12¼" × 6"). For international shipping, weight limits are more restrictive at 4 pounds for Small Flat Rate Boxes and Envelopes, and 20 pounds for Medium and Large Flat Rate Boxes. The package must close within the box's normal folds without bulging.

Can I use my own box for USPS Flat Rate shipping?

No, you cannot use your own packaging for USPS Flat Rate shipping. The Flat Rate pricing specifically requires using the USPS-provided Flat Rate boxes or envelopes, which are available free of charge at local post offices or can be ordered online from USPS.com. If you prefer to use your own packaging, you'll need to ship via regular Priority Mail, with rates calculated based on weight, dimensions, and destination zone rather than the fixed Flat Rate price.

Can I ship internationally with USPS Flat Rate boxes?

Yes, you can ship internationally with USPS Flat Rate boxes through Priority Mail International Flat Rate service to over 180 countries. International Flat Rate shipments have different pricing and lower weight limits than domestic service: 4 pounds maximum for Small Flat Rate Boxes and Envelopes, and 20 pounds for Medium and Large Flat Rate Boxes. These shipments include up to $200 in insurance and tracking capability. You must complete appropriate customs forms and verify that your items comply with destination country import regulations.