Thanks for helping the plugin get better!
This project uses GIT (github) for version control. SVN is required for wordpress plugin, but only the master can change it.
To contribute to the project, create a new Branch for your changes. Then, make a pull request.
try always to document the changes and new features, and always try to support the old features.
git repo:
https://github.com/viewup/up-wp-cart
SVN repo:
https://plugins.svn.wordpress.org/up-wp-cart/
for GIT/SVN reference, see https://gist.github.com/kasparsd/3749872