I ordered a large Walk in Run but in the basket, it always reverts to 3x4x2

Our largest pre-configured size for the Walk in Run is 3 x 4 x 2.

If you design a larger run in the configurator, it will show in your basket as 3 x 4 x 2 plus the additional extensions to achieve the size you require.

In most cases, this is additional Pack Gs.