The clearance charges are basically for handing the paperwork on the UK side. Someone has to officially 'receive' the goods at the port, sign papers, make sure duty/taxes (if any) are paid, and arrange the road transport. Aside from duty, this MAY be included in your quote, but if by any chance the mattress is held in customs, then there's additional charges which can add up very fast.
A single item is going to be stuffed into a container wherever it fits best for the shipper. You have no control over how it's handled or treated. I would still strongly advise putting the money toward a mattress here - if a major brand, you will find the equivalent - just don't go to Bensons for Beds or Dreams; talk to a small stockist or deal with a large store like John Lewis. If you have a Tempurpedic mattress it might survive. If you have one from The Original Mattress Company ..... feels good for 6 months or so, then it's all downhill from there in my experience.
Look at your brand and contact the manufacturer to see what the equivalent is here.
