DSF

just another option, I use the 5x5x5 from packaging monkeys on ebay, they work out around 14p each but you do still need to bubble wrap/loose fill them to take up the space.

http://http://www.ebay.co.uk/itm/QUALITY-SINGLE-WALL-POSTAL-MAILING-CARDBOARD-BOXES-/370846058444?var=640101643095

Also Nikki at sublimetals sells these