Sent a 20" haro mini this week.
got a bike box from lbs, front wheel off, stem/bars off, pedals off, drop in the box, pad out and tie the loose bits up, trim box tight to bike size.
cost £12.90 insured for the £80 sale price, Parcel2go, yodel 48hr, take pics of packaging during and after wrapping though, helps if it goes tits up!!
MM