Check out https://github.com/benalavi/buildpack-packages which is a more up-to-date way to install s3cmd (and other simple binary packages) on Heroku.

Heroku buildpack that installs s3cmd.

You probably want to compose this with other buildpack(s) using heroku-buildpack-multi (https://github.com/ddollar/heroku-buildpack-multi):

Set your buildpack:

heroku config:set BUILDPACK_URL=https://github.com/ddollar/heroku-buildpack-multi

And add this to your .buildpacks, i.e.:

https://github.com/benalavi/buildpack-s3cmd
https://github.com/heroku/heroku-buildpack-ruby

Once built you will have s3cmd available on PATH.

ruby test/buildpack_test.rb

There is also a Vagrantfile which will set up a VM with the build/test environment ready to go:

vagrant up
vagrant ssh
cd /vagrant
ruby test/buildpack_test.rb

The s3cmd-1.0.1 package this buildpack installs is available at https://s3.amazonaws.com/buildpack-s3cmd/s3cmd-1.0.1.tar.gz.

It is simply a repackaged version of the one available at http://sourceforge.net/projects/s3tools/files/s3cmd/1.0.1/ with only s3cmd and required S3 folder included.