Simple answer - you don't need to tell us that you've paid your bill!
If you've just paid your bill online, we'll get it shortly.
Even if you've popped a cheque in the post, you don't need to call us. As soon as we receive the cheque, we'll note it against your account. If you've paid your bill but still received your reminder, don't worry. Most likely, it'll have crossed your cheque in the post.
Please be assured that, however you've paid your bill, you don't need to spend your time telling us that it's on its way.
However, if you really want to tell us, just sign into Billing & payments and follow the Tell us you've paid links.