You could bury them in Africa.
Or, more usefuly, use them as birdscarers for crops.
The cost of sending CDs in the UK now depends on perception of size rather than weight.
Or, to be more exact, whether in the eyes of whoever's behind the post office counter, a CD-sized jiffy bag constitutes a 'small' or a 'large' letter.
The latter is twice the price of the latter.