The term Automated Flat refers to mailpieces that are capable of moving through the USPS’s high-speed Flats Sequencing System (FSS), an automated sorting process that enables more efficient handling and delivery.

Because these items can be processed mechanically rather than manually, USPS offers discounted shipping rates to businesses that design their mailpieces to meet its automation standards.

In simple terms, “flats” are larger than letters but smaller and thinner than parcels. The USPS defines very specific dimensional and physical requirements for these items.

To qualify, each mailpiece must meet these size requirements:

• Minimum: more than 6 inches long, or more than 5 inches high, or more than 0.01 inches thick.
• Maximum: 15 inches long, 12 inches high, and 0.75 inches thick.

The piece must be rectangular, and its length must be at least 1.3 times its height, ensuring the shape feeds smoothly through the automation equipment.

Thickness and flexibility are two key factors to consider. Each flat must have uniform thickness, meaning the contents cannot create bumps or protrusions greater than a quarter of an inch, and the item should bend easily without damage when subjected to USPS flexibility testing.

The weight limit varies by mail class: 13 ounces for First-Class Mail, less than 16 ounces for USPS Marketing Mail, and up to 20 ounces for Periodicals and Bound Printed Matter.

Source: EcoEnclose

Equally important are the restrictions on surface features. Clasps, strings, buttons, staples, or any other closure devices that could catch in machinery are prohibited. Tapes and seals must be smooth and non-reflective, and adhesives cannot interfere with barcode scanning. If your mailpiece includes inserts, they must be securely attached so they don’t shift around during processing.

For many brands whose products qualify for this program but whose shipping packaging currently does not, this means rethinking shipping solutions: moving away from bulky mailers or boxes and toward thinner, structured envelopes that maintain a consistent shape and thickness. In doing so, they not only comply with USPS automation requirements but also lower material weight, which further reduces emissions and shipping costs.

The USPS Automated Flats category rewards efficiency. By reducing manual handling, the Postal Service saves time and money, and it passes a portion of that savings along to participating mailers.

For small e-commerce brands shipping items like apparel, accessories, stationery, beauty samples, lightweight books, or similar products, the savings can be substantial. Even a dollar or two per order translates into tens of thousands of dollars in annual cost reductions for many companies.

Source: EcoEnclose

Navigating USPS’s technical requirements can be daunting. EcoEnclose makes it easy.

Our packaging engineers and sustainability experts specialize in designing lightweight, eco-friendly mailers that meet USPS physical standards for automation while aligning with your brand’s sustainability goals.

Here's what we bring to you.

Tailored Solutions

For brands exploring the Automated Flats category, EcoEnclose offers both off-the-shelf and customized solutions that strike a balance between structure, flexibility, and environmental performance. We advise on packaging materials and formats — from recycled paperboard mailers to ultra-slim padded envelopes and engineered inserts that maintain uniform thickness — to ensure your shipments meet the requirements.

Logistics Partnership

Our team bridges the gap between packaging and postal logistics. We help you prepare the technical documentation and mailpiece samples required for USPS pre-approval, connect you with trusted mail consolidators, and guide you through the approval timeline from start to finish.

Sustainability Experience

Beyond compliance, EcoEnclose brings deep sustainability expertise. We help you achieve efficiency in the most environmentally responsible way possible by using recycled, recyclable materials — delivering packaging that is compliant, cost-effective, brand-aligned, and planet-forward.

Testing Partnership

We also work with brands to prototype, test, and optimize mailers before large-scale rollout, ensuring durability, flexibility, and consistent performance in transit. With our experience working closely with USPS and logistics partners, we know the subtle details that make the difference between approval and rejection.

→ Connect with EcoEnclose Packaging Experts to get started.

Do Automated Flats include shipment tracking?

Not in the same way standard USPS parcel services do. Automated Flats can come with limited “tracing” rather than full end-to-end tracking. This means you won’t have detailed visibility into a package’s movement after induction. For brands that rely heavily on real-time tracking to support customer experience, this may be an important limitation to weigh.

What kind of approval is required to qualify for USPS Automated Flats?

Because Automated Flats are processed through specialized USPS machinery, each mailpiece design must be pre-approved by the USPS Pricing and Classification Service Center (PCSC). This ensures your packaging meets all physical standards and can move smoothly through automation equipment.

The approval process typically takes three to six weeks, depending on USPS volume and design complexity.

Most brands work with a mail consolidator, who manages classification submissions, approvals, and compliance documentation. EcoEnclose helps coordinate this process — from preparing technical specifications and samples to guiding you through each approval step.

What types of products qualify for Automated Flats?

Automated Flats are ideal for thin, lightweight, and flexible items — such as light clothing like socks, ties, undergarments, cosmetics, supplements, personal care, or home supplies, and small printed materials.

To qualify, your product must be:

• Under 5/8" thick

• Under 13 ounces (for First-Class Mail)

• Under 4" x 6", or flexible

Products that require more structure or protection — or are irregularly shaped — typically won’t qualify.

Is the transition to Automated Flats worth it for my brand?

For many direct-to-consumer brands shipping small, lightweight items, yes. Automated Flats can yield meaningful postage savings, reduce material usage, and streamline fulfillment. While there are tradeoffs, the cost and sustainability benefits often outweigh them.

And with EcoEnclose’s expertise in design, testing, and USPS compliance, you can maintain a polished unboxing experience while lowering your shipping costs and environmental footprint.

→ Connect with EcoEnclose Packaging Experts to get started.