Plug in the back of your controller, replacing the battery cover and batteries. No need to buy any more Duracells. You recharge it just like you recharge a ps3 controller via a mini usb to usb cable, which is provided in the pack. You do not need to remove the pack to recharge it.
For some reason, the black colour is £4.99 instore, and the white colour is £9.99. Both are £4.99 online but apparently are meant to be £9.99, but was told this could be due to a pricing error.
If it is a pricing error, grab them while you can, will save you money in the long run.