Sometimes contributions created on Macs have files and folders embedded in them with names like .DS_Store and __MACOSX. Those files are created by Finder.

The only way to not create them is to *not* use Finder to access the folders containing those files.

Free tool to clean out those files:

An alternative app similar to Finder is PathFinder. PathFinder is probably the easiest tool to utilize for dealing with this topic.
While not free, is known to work well for allowing access to such folders/files without creating the .DS_Store files, and allowing easy removal of those files if found, and then allowing zipping via simple right-click menu options etc. However, using the cleaner tool above is a free way to avoid the issue.

Some ideas and explanation of the topic:

Additionally, since those files are hidden in normal Finder mode, to see whether the files exist, you can turn the display of hidden files on/off by running this command from a terminal window:
defaults write AppleShowAllFiles -ON
defaults write AppleShowAllFiles -OFF
Or, if your version of Mac OSX supports it, there is this free tool for turning on/off the display of hidden files in Finder: