What Are Poly Mailers? | Shipping Bags Explained

Poly mailers are lightweight plastic shipping envelopes made from polyethylene, designed for sending non-fragile items like clothing and documents.

If you run an online store or ship packages regularly, you’ve likely encountered poly mailers. These flexible plastic envelopes are the workhorse of ecommerce shipping for soft goods. They weigh a fraction of what a cardboard box weighs, which cuts postage costs, and they take up far less storage space in your warehouse or closet. Here’s what you need to know about them, from sizing to materials to common mistakes.

What Are Poly Mailers Made Of?

Poly mailers are made from polyethylene — a durable, flexible plastic. Most commercial options use low-density polyethylene (LDPE) or a blend of linear low-density and low-density polyethylene (LLDPE/LDPE). This material gives the mailers their signature qualities: lightweight, thin but surprisingly tough, and resistant to moisture.

Most poly mailers are opaque, which keeps the contents hidden from curious eyes during transit. They typically come with a self-sealing adhesive strip along the flap, so you don’t need tape to close them — just peel and press. The standard thickness across the industry is 2.5 mil, though suppliers sell variations from 2.35 mil up to 4.0 mil for heavier-duty needs.

What Are Poly Mailers Used For?

Poly mailers are ideal for anything that isn’t fragile. The most common uses include:

• Apparel: T-shirts, hoodies, jeans, jackets, and other clothing items
• Soft goods: Blankets, towels, and linens
• Documents: Contracts, books, and paperwork that need protection from the elements
• Accessories: Jewelry, small accessories, and phone cases

Businesses prefer poly mailers over boxes for a simple reason: they’re lighter. Shipping costs are often based on weight, and a plastic mailer weighs just a few grams compared to the 200+ grams a cardboard box might add. They also store flat, meaning you can keep hundreds on a single shelf.

Because they’re moisture-resistant, poly mailers offer decent protection from rain and spills during transit. That said, they aren’t waterproof hard cases — a sharp object or heavy crush force can puncture or damage them.

How To Size Poly Mailers Correctly

Getting the size right matters more than you might think. An oversized mailer wastes money and looks sloppy; an undersized one makes packing a struggle. Poly mailers are measured in width × length, and the adhesive flap does not count toward the stated pouch dimensions.

Here’s how to pick the right size for your product:

• Measure your item’s folded length, width, and thickness.
• Add the product’s thickness to the width and length to ensure comfortable fit.
• Leave a little extra room for easy insertion and proper sealing.

Standard flat sizes range from 6 × 9 in for jewelry and small accessories up to 19 × 24 in for bulky coats and blankets. In between, you’ll find common options like 7.5 × 10.5 in for folded T-shirts, 10 × 13 in for sweaters, and 12 × 15.5 in for hoodies and jeans. Suppliers also sell smaller 4 × 6 in mailers and larger 24 × 36 in formats for oversized items.

If you’re shipping bulky items that don’t flatten well, consider gusseted poly mailers. These expandable versions have 2 to 5 inches of side pleat, letting you ship puffy jackets or thick blankets without overstuffing a flat mailer.

Common Poly Mailer Mistakes To Avoid

Poly mailers are simple, but a few recurring mistakes cause problems for new shippers:

Using them for fragile items. Poly mailers provide flexibility and moisture resistance, but they don’t absorb impact. Ceramics, glass, and electronics need bubble wrap, foam, or a rigid box — or at minimum, a poly mailer with internal protective padding.

Counting the flap as usable space. Size charts exclude the adhesive lip from the nominal dimensions, so don’t assume that extra inch is available for your product.

Overstuffing flat mailers. If your item bulges against the seal, the adhesive may fail during transit. Bulky products belong in gusseted mailers or boxes.

Assuming all poly mailers are recyclable. Recyclability varies by product, resin blend, and local facilities. Check the product documentation before making claims to your customers.

One more point: poly mailers are shipping containers, not food packaging or long-term storage bags. Don’t use them for food contact or to store items indefinitely unless the manufacturer explicitly says it’s safe.

For carrier acceptance and postage, verify current rules from USPS, UPS, or FedEx before shipping. These carriers accept poly mailers for soft goods, but size, weight, and machinability requirements still apply. If you’re ready to switch to an eco-friendlier option, our roundup of the best biodegradable poly mailers breaks down compostable and recyclable alternatives that hold up in transit.