Hi, Shirley.
 
Letting MOST of your shells rot out underground is a workable
plan in the tropical locales; plenty of organisms to take up
the bounty you are making available and soils which tend to be
porous and quick to drain -- along with high temperatures to
keep things moving right along.
 
The main caveat has already been given: glossy finishes can be
in peril. The freezer is a valid option for large cowries -- that have
had a chance to release most of their reservoirs of water so as not
to crack the shell as the animal freezes and expands.
 
For smaller shells, there is the method we often used in Guam, where
freezers were small and filled with more important things -- like ICE!
The best and least smelly way was to use a container of Baby Oil, as
in scented mineral oil.
 
It is IMPORTANT to have a small platform to hold the shells up off of
the bottom, where the "juices" will collect as the anaerobic actions
proceed. These are BAD "juices": smell bad and do bad things to the
finish of the shells. (A piece of window screen, folded up, or a
small PLASTIC pot-scrubber will work as platforms.)
 
Except for the shiny stuff, you've got most of the rest of it
figured out, so... GEEV 'UM!
 
Aloha,
 
makuabob (a.k.a. Bob Dayle)
 
More shell cleaning info can be found in the pages of
 
THE CAPTURED COWRY at  http://www.oaktree.net/makuabob/