Yes the weight is correct. But the shipping cost specified is absurd.
For example. One item 140gram 25x15x10 cm. It gives a price of £1.75. Royal Mail website gives it as £5.20. It also gives the price as a letter, when it is too big to be accepted as a letter. It also gives the option to send it airmail to a UK address for £2.38 which is obviously not a service available, but only £1.57 to send to Spain.
In other words the costs given have no connection whatsoever with the prices Royal Mail charge. The Royal Mail charge by size and volume as well as by weight. This could be where the problem lies. If I change the size of the product, it doesn't affect the price. And it would explain how it will allow a large box to count as a letter, as long as it is within the weight limit.
But if the Royal Mail extension doesn't reflect the cost of the service what's the point of it? Is there an extension where I can manually enter the delivery costs in the manner of cost A for first item + cost B for each successive item?