Many items over 80cm in length can be sent with Royal Mail for the usual low Royal Mail shipping charge (or free for some orders).
However, Royal Mail has a maximum parcel weight of 5Kg.
Therefore, if you buy multiple items, taking your basket weight above 5Kg, Royal Mail will no longer be available for your order. Your basket will default to DPD Delivery and DPD will charge a very significant premium for large items (over 80cm).
If you are buying multiple items, including an item over 80cm in length, it's often cheaper to place a separate order for the large item, selecting Royal Mail delivery. This will avoid the DPD "Large Item Surcharge". Then place a separate order for the remaining items.