My advice is, start saving commendations. When you hit level 50, you should have 1000/1000 commendations and then buy Champion bags.
You need 600 Warzone commendations to make 200 Mercenary commendations.
Champion bag cost 200/200 so you need 800 Warzone commendations / bag.
Allways try to do PvP dailies / weekly quests.
Its not worth it to buy those low lvl pvp items, when you need so many commendations after hitting level 50 (Mounts,Gear).