Length is the dimension that most often takes a parcel out of the published prices entirely, and it does it long before weight does. Royal Mail's largest published shape is a Medium Parcel at 61cm x 46cm x 46cm; anything longer has no published price anywhere in this record whatever it weighs. A three-metre item is not an extreme case in this trade, it is simply a different product, sold by quote and often on a pallet network rather than a parcel one.
Where length stops being a parcel
At 61cm on Royal Mail's published grid. Evri's published table prices by weight above 2kg and does not publish a length limit alongside it, and Parcelforce publishes weight bands only. So for anything long, the published prices answer the question only if it fits inside 61cm, and the honest answer above that is that you have to ask.
Why long items are priced badly by volume
Above the flat bands, carriers charge on the greater of actual and volumetric weight, and a long thin item has a poor ratio: it occupies a whole shelf length while weighing very little. Girth, which is length plus twice the width plus twice the depth, is the measurement most carriers use to catch exactly this shape, and a tube can fail on girth while passing on every individual side.
What actually carries a three-metre parcel
Usually a pallet network or a dedicated van rather than a parcel network, because the sorting equipment in a parcel hub has a length limit that is not negotiable. That changes the questions: whether the item can be collected from where it is, whether it needs two people, and whether the destination can receive it. Price is the last of those, not the first.
Questions people ask about send long parcel
What is the longest parcel I can send?
On published prices, 61cm, which is the longest side of Royal Mail's Medium Parcel. Beyond that no company in this record publishes a price and the service is quoted.
How much does a 3m long parcel cost to send?
It is quoted rather than published, and usually carried on a pallet network or by dedicated van rather than a parcel network, because parcel sorting equipment has a hard length limit.
Why is a light long item expensive?
Volumetric weight and girth. A long thin item occupies shelf length while weighing little, and girth is the measurement carriers use to catch that shape.