What can I do with leftover commendations?
Solution 1:
With Patch 2.0, planetary commendations all got combined into a single type. Any commendations you don't immediately spend at the Korriban vendor can be saved and spent at the Dromund Kaas vendor after a couple more levels, and so on, all the way up to Makeb.
The commendations vendors are located on the appropriate planet or on the Fleet (in the area of the map marked "Supplies").
Once you've finished levelling, the best idea is to buy the best lockbox you can afford (e.g., the purple one from the Makeb vendor) with the commendations you have left.