First bounce all tracks down with effects etc. applied, unless you are certain you are both using exactly the same plugins and the plugin settings aren't yet at the final settings. In which case you'll also need to save the plugin settings as presets as well so you can send the presets to your collaborator.
Include bouncing of soft synth tracks to audio.
Export each track as a seperate file, I suggest the best format being "broadcast wave". A broadcast wave contains a time stamp which tells the DAW it's being imported to where along the timeline the track is to be placed. Though if a Pro Tools user sends you a broadcast wave be aware it might get placed an hour later on your timeline than it was on there's (seems to be a bug in Pro Tools that one).
Title each wave file with a meaningful name so they know what it is and encourage them to do the same when sending you stuff.
You won't save much space by zipping the files, but zipping them into a single zip file does mean all the files relevant to a particular project are together as a bundle.
Then upload them where they can be found by your collaborator or email them across.
Another option is to do a rough mix of what you're working on, bounce to a stereo wave file and send them that. That way they can add further tracks of their own and repeat the process, sending you a bounce of their latest version and so on. To further save on data transfer you could even mp3 your rough bounces rather than transfer stuff as wave files. Though at some point someone's going to need a complete high quality set of the audio stems (tracks) to do the final mix.
It helps some if you're both using the same software so you can bundle up the entire project and send that, but again you also need to be using the same plugins. A fast internet connection with an unlimited or at least very high data allowance is also advisable.
I'm unaware of any file depository that takes uploaded stems and does a stereo mix for you. I suspect yoh may need to check your export settings in Sonar because it sounds like you might have exported a stereo remdering from the master bus instead of single tracks (assuming a collection of tracks is what you wanted to send).
If this depository does do mixes for you is it any good at it? Might be a huge time-saver if it is,,,, :-)