Mailing bags are one of those products that most people have received but fewer have thought much about. They show up on your doorstep wrapped around a t-shirt or a pair of trainers, get cut open and binned, and that's usually the end of the interaction. But if you're buying them in bulk for a business or trying to work out which type is right for what you're shipping, there's actually a reasonable amount worth knowing.
This guide covers what mailing bags are, the different types available, the terminology that causes confusion, and the question that surprisingly many people search for: why do mailing bags smell?
What are mailing bags?
A mailing bag is a self-seal polythene bag designed to protect items during postal transit. They're made from low-density polyethylene (LDPE), a flexible plastic that is lightweight, waterproof and tough enough to withstand normal handling through the postal system. The seal is a peel-and-press adhesive strip, usually covered by a backing tape that you remove before closing the bag.
Mailing bags are used to ship clothing, accessories, books, flat-packed items, documents and most other goods that don't require rigid protection. They're the packaging of choice for the majority of small parcel shipments in the UK because they add minimal weight and cost, take up very little storage space and are quick to pack.
What are shipping bags called? A note on terminology
Mailing bags go by several names, and the terminology varies enough to cause genuine confusion when shopping around. Here is what the different terms mean in practice.
Mailing bags is the most widely used trade term in the UK and what most packaging suppliers will use. Postage bags is the term many individual sellers and casual users reach for, particularly those sending parcels through Royal Mail. Parcel bags is used interchangeably with both of these. Shipping bags leans more towards American terminology but is common in e-commerce circles and refers to the same product. Poly bags or poly mailers describe the same type of bag, with poly referring to the polythene material.
There is no meaningful technical distinction between these terms. They all describe the same category of product: a self-seal polythene bag for sending items through the post. The name that appears on a supplier's website is largely a matter of convention. On Mr Bags, the full range is listed under mailing bags.
Types of mailing bag
Within the mailing bag category there are a few distinct types, each with slightly different properties and best uses.
Standard polythene mailing bags
The most common type. Made from LDPE, available in grey, white or black, and sold in a wide range of sizes from small enough for a single t-shirt to large enough for a coat. Grey is by far the most popular colour for UK sellers, particularly those on Vinted, Depop and eBay, because it's opaque, neutral and looks clean and professional without any branding.
Plastic mailing bags with permanent seal
Some mailing bags have a permanent seal rather than a returnable one. These are intended for single-use dispatch and cannot be resealed once opened. Most standard mailing bags fall into this category. Bags with a double seal strip allow the recipient to reseal the bag for a return, which is useful for businesses that offer easy returns and want to keep the packaging process simple for customers.
Biodegradable and compostable mailing bags
Made from plant-based materials rather than fossil-fuel-derived polythene, these break down under industrial composting conditions. They look and feel similar to standard bags in use but require a different disposal route. They cannot be recycled with standard soft plastics and should not go in a home compost bin, as they need the higher temperatures of an industrial composting facility to break down properly.
Recycled polythene mailing bags
Bags made from recycled LDPE rather than virgin plastic. They perform the same function as standard bags and are recycled via the same soft plastics route at end of life. The difference is in the production stage, where using recycled rather than virgin material reduces the overall environmental footprint of the bag.
Padded mailing bags
A polythene outer with a layer of bubble wrap or foam padding inside, used for items that need some protection from impact rather than just waterproofing. Useful for electronics, fragile accessories, glasses, jewellery and similar goods that would travel safely in a padded envelope but don't need the full protection of a cardboard box.
Why do mailing bags smell?
This is the question that brings a lot of people to this page, and it has a straightforward answer.
Not all mailing bags smell. New mailing bags made from virgin polythene have a very faint plastic smell at most, and many have no noticeable odour at all. The distinctive sharp or acrid smell that some mailing bags have is almost always a sign that the bags were made using contaminated recycled polythene.
Here is the process that causes it. When polythene bags are recycled, the material is melted down and reformed into new pellets, which are then blown into new bags. If the waste polythene fed into this process contains contaminants, those contaminants end up in the new bags. Common sources of contamination include residual printing ink from old printed bags and films, and adhesive residue from tape and label glue lines on packaging that wasn't cleaned before recycling.
When these contaminated pellets are reprocessed at high temperatures, the chemical residues from the inks and adhesives produce volatile compounds that off-gas into the finished bag. The result is that acrid, smoky smell that some people describe as resembling cigarette smoke or burning plastic. It transfers readily to the items packed inside the bag, which is a particular problem for clothing sellers whose customers may notice the smell when they open their parcel.
This is fundamentally a quality control issue at the recycling stage. Reputable suppliers who use clean, uncontaminated recycled feedstock or virgin polythene do not produce bags with this problem. If you have experienced smelly mailing bags, it is almost certainly a sign that the bags were produced using low-grade recycled material where corners were cut on contamination control.
The practical advice is to buy from a supplier with consistent quality standards and, if possible, to open a new pack and check before committing to a large order from an unfamiliar source. If you're sending clothing, a bag that smells will reflect on your business regardless of what's inside it.
What colours do mailing bags come in?
Grey is the most popular colour and what most people picture when they think of a standard mailing bag. It's the default for the majority of UK clothing sellers and general e-commerce businesses.
White mailing bags are also widely available. They look cleaner and brighter than grey, which some businesses prefer for aesthetic reasons, but they are not fully opaque in all thicknesses, which means the outline of the contents can sometimes be visible through the bag. For most practical shipping purposes this doesn't matter, but it's worth knowing if you're shipping anything you'd prefer to keep entirely private.
Black mailing bags are less common but available. They offer complete opacity and a more premium look than grey or white. They're used by businesses that want a distinctive, considered packaging aesthetic.
Beyond these standard options, custom printed mailing bags can be produced in a range of colours with your branding printed directly on the material. This is covered in more detail in our guide to printing on mailing bags and custom branding.
Where to buy grey mailing bags and plastic mailing bags in the UK
Grey polythene mailing bags and standard plastic mailing bags are available from packaging suppliers, office supply retailers, stationery shops and online marketplaces. The range, quality and pricing varies considerably between these sources.
For the widest selection of sizes and the best unit pricing, buying directly from a specialist packaging supplier is the most sensible route for anyone sending more than a handful of parcels a month. Mr Bags stocks grey mailing bags across a full range of sizes, from small options suitable for a single t-shirt through to large bags for coats and bulky soft goods. You can browse the full range on our mailing bags page.
If you're looking for where to buy plastic mailing bags locally, post offices, WHSmith and Ryman stock small packs. These are useful if you need bags urgently, but the unit cost is significantly higher than buying in any meaningful quantity from a supplier, and the size range is limited. For anything beyond occasional one-off purchases, buying online in bulk from a packaging supplier will save you money and ensure you always have the right size in stock.
How to choose the right mailing bag
Three things to get right: size, thickness, and quality.
For size, fold the item you're shipping flat, measure it, and add around 50mm to each dimension. This gives you enough room to get the item in and seal the bag flat without the contents pressing against the adhesive strip. A more detailed breakdown by garment type is available in our mailing bag size guide.
For thickness, a minimum of 50 microns is a reasonable baseline for standard clothing and soft goods sent through the postal system. Anything thinner risks splitting under normal handling. Heavier items warrant a thicker bag. If a supplier doesn't list the micron rating, that's worth treating as a reason for caution.
For quality, buy from a supplier with consistent standards. As covered above, the smell test is a reasonable proxy for quality: a bag that smells of burning or chemicals is one where the recycled feedstock was contaminated. A good supplier using clean materials produces bags that have no significant odour.
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