Yes thats correct it is a one off payment, if you do not use more than Is stated per month then it will NOT cost you one more penny, IF you go over the limit in one month then the over the limit amount will be deductd from the £15 you first paid for when you ordered the sim, you can top up this sum,when you wish. They say that they sometimes send adverts to you, but I have never found this intrusive in fact I only remember seeing two adverts in two months. Well worth giving it a go. As I said I have no complaints with the service at all and have recommnded it to my friends. Maybe not the right deal for a heavy user but as you stated that you are a light user then I think this will suffice.I thought it was too good to be true also, but as I say had it for a while now and no complaints at all. Even their roaming prices are very good. Have a god read on their site.