As a Brit, I'd associate "posting" with the act of sending a physical object via a postal courier.
"Mailing" is more immediately associated with email.
"Disk" and "Disc" originally meant the same thing, being an object of cylindrical profile where the diameter is much greater than the thickness. In modern usage, "disc" tends to refer to optical storage devices such as CDs, DVDs and Blu Rays, whilst "disk" tends to be used for magnetic or solid-state storage mediums. Either way, I would find it slightly weird to "mail a disc/k". Either word is acceptable for other objects.