A Royal Mail Large Letter can be no thicker than 2.5 cm, and a padded paper mailer can push a parcel past it. Why the outer layer is the last swap to make, and how to measure it first.
A Royal Mail Large Letter can be no more than 35.3 cm long, 25 cm wide and 2.5 cm thick, and can weigh no more than 750 g. T-shirts, paperbacks, prints and phone cases routinely go out inside those limits. They are also the limits a seller is most likely to break when plastic packaging is replaced with paper.
Royal Mail’s second class prices show what is at stake. A Large Letter between 101 g and 250 g costs £1.90, and between 251 g and 500 g it costs £2.40. A second class Small Parcel costs £4.40 bought online. Crossing the 250 g line adds 50p. Leaving the Large Letter format, for a parcel that was in the 101 g to 250 g step, more than doubles the postage.
Plastic usually enters a parcel in four places: the outer mailer, a clear bag around the product, bubble wrap or air pillows, and the tape. The mailer is the one most likely to change the format a parcel is priced in, which is why the mailer should be the last thing a store changes, not the first.
Start inside the parcel
The clear polybag around a garment or a print is the easiest swap to make. Glassine is paper that has been run through heavy rollers until it turns smooth and semi-translucent, so a customer can still see the product through it while it keeps off dust and fingermarks. It adds a few grams and almost no depth, so the parcel stays in the same format and usually in the same weight step.
One detail decides what a seller can then say about it. Some glassine is waxed for moisture resistance, and many councils exclude waxed paper from household paper collections. Cornwall Council and Colchester City Council, for example, both tell residents to keep it out. A waxed glassine bag needs specialist recycling, and the insert card should not suggest otherwise.
“Ask for the specification to say waxed or unwaxed in writing,” says Simmy Lin, General Manager of Bk-bags, a Ho Chi Minh City based supplier of reusable shopping bags and e-commerce packaging. “The two look almost identical in a sample pack. The difference only matters when you write the disposal line on your insert, and by then the stock is already on the shelf.”
Then the tape
A paper mailer sealed with polypropylene tape is a paper mailer with plastic on it. There are two paper alternatives. Self-adhesive paper tape fits the hand dispenser already on most benches. Water-activated gummed tape holds better on heavier boxes but needs its own wetting machine.
Ask which version a supplier is quoting. Reinforced gummed tape carries glass-fibre strands for strength, which is a different material from the paper around it.
Bubble wrap: a question of shelf space
Honeycomb paper is die-cut kraft that opens into a mesh of cells when it is stretched. Pulled round a product, the cells lock into each other, so the wrap stays closed on its own. Expanded, it adds depth, so inside a Large Letter it needs the same thickness check as the mailer.
The bigger difference is in the storeroom. A roll of bubble wrap takes up its full volume from the day it is delivered. Honeycomb paper is delivered as flat sheets and only takes up its full volume once it is round a product. For a seller paying for a unit, a garage bay or space in a fulfilment house, storage is the stronger argument of the two. For a seller with room to spare, the case is weaker and price per metre decides it.
There is also a tax angle, though it rarely lands on a small retailer directly. Plastic Packaging Tax has been £228.82 per tonne since 1 April 2026. It is charged on plastic packaging components made with under 30 per cent recycled plastic, and the business that pays is the one that makes or imports the packaging, once it reaches 10 tonnes in 12 months. A seller bringing in its own bubble wrap and mailers by the container should check which side of that line it sits on.
Honeycomb’s weak point is a sharp corner. The cells soften a drop, but a heavy item with hard edges can push straight through them, and those products still need a box.
The mailer: measure before you order
This is the swap that can move a parcel between price formats, so it goes last and it gets measured, in three ways.
Packed thickness. Measure the mailer with your thickest best-seller sealed inside, at its fattest point. A padded paper mailer gets its protection from layers of paper, and layers take up depth. If the packed parcel goes over 2.5 cm, Royal Mail prices it as a Small Parcel, at £4.40 instead of £1.90 or £2.40.
Width, if you use Evri. Evri’s postable service, its letterbox format, takes items up to 23 cm wide, 35 cm long and between 1 cm and 3 cm deep, weighing from 250 g to 1 kg. That is 2 cm narrower than a Royal Mail Large Letter, so a mailer chosen to fill Royal Mail’s limit will not fit Evri’s. Evri otherwise prices parcels by weight, with its first band running up to 1 kg, so a few extra grams of paper rarely change an Evri price.
Weight against the step. Suppose a T-shirt in a poly mailer weighs 240 g and travels as a £1.90 Large Letter. If the paper mailer of the same size adds 20 g, the parcel crosses 250 g and costs £2.40. Weigh your three best-selling products packed in the paper version, not the empty envelope.
For anyone choosing between formats, this comparison of paper and plastic mailer formats shows which are built for flat goods, which for cushioning, and how each sits against carrier bands.
“Put a sealed sample with your bulkiest product through a letter-size gauge before the order goes in,” Lin says. “An empty envelope tells you almost nothing. The parcel your customer receives is the one the carrier measures.”
Paper mailers are not waterproof. A parcel left on an open step in heavy rain can arrive soaked, and liquids still need a plastic barrier.
What you can say on the pack
Once the materials change, the natural next step is to tell customers. The Competition and Markets Authority’s Green Claims Code sets out that environmental claims must be truthful and accurate, clear and unambiguous, and backed by evidence. Since the Digital Markets, Competition and Consumers Act 2024 took effect in April 2025, the CMA can fine businesses directly, without going to court, and its own guidance puts the ceiling at 10 per cent of worldwide turnover.
The CMA’s January 2026 guidance on supply chains adds that the manufacturer and the retailer may both be liable for a misleading claim. A seller who repeats a supplier’s “plastic-free” line on a product page owns that line.
Two habits protect against it. First, get a component list from the supplier that names the material of the outer layer, the lining, any window and the tape. A kraft mailer can have polyethylene bubble film bonded inside it and still look like an all-paper envelope from the outside. Second, describe what the pack is made of rather than what it achieves: “paper mailer, paper tape” can be checked, while “eco-friendly packaging” cannot. The same component list will be needed anyway, because UK packaging extended producer responsibility asks obligated businesses to report packaging by material and weight.
Trial one product line before you reorder
Order samples before setting a start date. Bk-bags, which produces in Vietnam and China and has been supplying packaging since 2006, gives sample lead times of 10 to 14 business days, so a request sent this week reaches the bench in two to three weeks.
Then change one product line, not the whole catalogue. Time how long a parcel takes to pack, log damage reports, check postage per parcel against the Royal Mail or Evri price list you actually use, and keep any customer messages about the packaging.
“The inner bag and the tape change nothing at the counter, so they can go this month,” Lin says. “The mailer changes the format your parcel is priced in. Give it a week of real orders through the gauge before you reorder in volume.”
If packing time holds and postage per parcel does not move, the switch is ready for the next product line. If either one moves, the measurements show which layer to change back.
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