@KLG all you are buying is a voucher online instead of going to a retail location and buying one there. all it will do is put funds on your account in the value you buy. once you have funds on your account you can do what ever you want.
you can buy a 1gb data addon for $30
you can do an immideate plan change to a plan taht is different than your current plan. (you forefit anything remaing of your existing plan and start the new one right away