Sorry but I'm a bit of a noob on planetside 2 but if you spend 200 certs to buy c4 or whatever and then use it, does it go away? Like does it only have one use? It seems overpriced if it was like that...
You can use it multiple times, but 1 x C4 charge costs 100 infantry resources. The maximum capacity for those resources is 750 so you can use them 7 times without needing to wait. Of course you have to buy them at the infantry consoles. The same goes for grenades, med-kits, mines etc.
Just to clarify that a bit. You can stock up to 40 C4 in total, but of course can only buy 7 at a time if your inf resources are full.
Pro-tip: You can also resupply them from the load-out screen on your regular menus (Hit esc). That way you can stock up when you're out in the field and just realized you're sitting at 750.