The Poly Mailers

Shipping custom mailer orders to the United Kingdom

Poly Mailers for London Businesses

UK postal pricing is unusually sensitive to one measurement: how thick a parcel is. There is a format band for items that fit through a letterbox, and staying inside it or falling outside it changes both the price and whether a delivery succeeds first time.

For London businesses shipping small items in volume, that single threshold is often the largest cost lever available, and it is decided almost entirely by packaging choice.

Quote an order for London

Tell us what you ship and where it goes. We confirm the specification, the schedule and the price in writing.

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Who buys mailers here

Sectors where poly mailers are genuinely the right packaging, with the reason each one appears rather than a generic list.

  • Small-item ecommerce

    Jewellery, cosmetics, accessories and print — the categories that can realistically stay inside a letterbox format.

  • Fashion and boutique retail

    A dense independent retail base with high presentation expectations.

  • Publishing and print

    Flat goods where the thickness question is decided by the item rather than the packaging.

What we would suggest

Thickness, not weight, is usually the deciding measurement

Most people optimising shipping cost start with weight. For small UK parcels the more consequential measurement is often depth, because a format threshold sits at a specific thickness and crossing it moves the item into a different pricing band entirely.

That is why a slightly larger mailer can be cheaper to send. A folded item lying flat in a bag with a bigger footprint stays thin; the same item compressed into a smaller bag becomes a compact lump that no longer fits through a letterbox.

Check the current thresholds with your carrier rather than relying on a figure published anywhere, including here — they are reviewed periodically. Then measure a real packed parcel against them rather than estimating.

Packing to stay flat

  • Choose footprint over compression

    A wider, flatter parcel frequently costs less to send than a smaller, thicker one.

  • Fold to a rectangle

    An irregular bundle has a thick point, and the thick point is what gets measured.

  • Rethink the insert

    A folded card, a sample and a returns slip can be the millimetres that cross a threshold.

  • Measure a packed parcel, not the product

    Flat product dimensions tell you nothing about what it becomes once bagged.

When it will not fit through a letterbox

Some products simply cannot stay inside a letterbox format, and forcing the attempt costs more than accepting it. Where an item is genuinely a parcel, the packaging decision shifts back to fit and protection rather than thickness, and the right approach is the ordinary one. The method for working that out from a packed sample is set out in the size selection framework.

Questions from London buyers

What size poly mailer fits through a UK letterbox?

It depends on the current format thresholds set by your carrier, which are reviewed periodically, so check them directly rather than relying on a published figure. Then measure a real packed parcel rather than the product on its own.

Is a smaller mailer always cheaper to send?

No, and this is the most common mistake in UK small-parcel shipping. A tighter bag makes a folded item thicker, and thickness is what most format thresholds are built around. A wider, flatter parcel often costs less.

Do you supply polythene mailing bags in the UK?

Yes. Orders are produced to your specification and shipped to your UK address; we hold no local stock, so we confirm the schedule in writing rather than implying next-day availability.

Size for a postal threshold

Send us a packed parcel’s dimensions and we will suggest a format that keeps it in the cheapest band it can reach.

Ask about letterbox sizing
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