I noticed Amazon are selling a 15kg bale of Fitch in a box
Can't see it on the Fitch website so it sounds like Amazon have picked a size in between 10kg and 20kg and ship it in one of their boxes! However it costs slightly more than 20kg direct from Fitch, without a box. Personally I like it in a plastic bag as can squish it into different shapes.
https://www.amazon.co.uk/Fitch-Paper...Fitch+in+a+box