Purchased Cartel Coins to hide my helmet, but it still wants more.

Indeed, as proxos666 said, unlocks like new inventory slots and hide head slot options, can be purchased in the Cartel market, and then need to be used, similar to using an EXP boost or opening a crate. 

Once it's been used from the inventory, it'll unlock the option for your character. 

If you don't see it in your inventory, check the Cartel market window. It has a little 'holding' menu where certain items can be refunded, it might be there.