If its something like that, staples have some archive boxes that once you've done GCSE Origami make sense!
They are certified to 25kg and also dont exceed any size limitations, becasue when calculating shipping weight its volumetric weight that matters..
So you might be able to send a 1 metre cubed box weighing 5 kg, but the system might only let you send an 80cm cubed boxed at 20kg if you get my drift