Probably won't have much visibility now the item is sold but the definitive statements about postage costs are somewhat predictably, inaccurate.
Paisley Freight who was used in this case, advertise @ £42.99 for a bike. Entering the box dimensions and weight is highly likely to give a price below this, based upon all the varialbes I tried when quoting.
The weights I tried made zero difference to pricing, size of the package did have an effect. I ended up with a box rather longer than needed or desired, which bumped up the cost from what I'd expected to be a fraction under £30 to just over £36.
Those who said parcelforce 48 could be done for 20 quid were presumably discussing a kids bike. It was over 3 times that for the bike in this case.