I’m sure there used to be a whole thread on Retrobike about the best companies to use for posting/couriering a bike but I’ll be buggered if I can find it!
Anyone got the link? Or better still, the cheapest most reliable company to use?
In the UK I've used Paisley Freight and Parcelforce without any problems (check if Parcelforce are still doing their 48hr "Large" service though, due to Covid19).
Get a cardboard bike box for free from Halfirds/bike shop.
Paisley Freight have an option where they supply a box and packing, I once sent this to the seller, who packaged it up and it was collected a few days later. Or you could just send an empty box to a seller, then arrange the collection.