They last for one match. Whichever match you buy them beforehand, after the match is over, they will be gone.
And you can use them however. Explosive Expert can help in certain situations, having an extra grenade slot. If you want to be extra stealthy, Untraceable will help. Not firing tracers means people can't see where your bullets are coming from. I think there's a couple others but I don't pay much attention to them.
They are for one match only. As previously stated explosive expert for the grenading players who feel one at a time isnt enough. untraceable for the keen on hiding in the shadows. Bargain is most helpful to those using the higher medal kickbacks (creepy crawler for example) to reduce the amount needed by one. And the last one is ping, this always you to spot anyone that is in short range of you to give you a slight advantage. Explosive expert is $25,000 Untraceable is $50,000 And both bargain and ping are both 100,000. I hope this helps you out