I've been a Sprint customer over six years and have recommended several people to Sprint!!! The Sprint site has ALWAYS had issues and now the bill pay and view bill after SEVERAL times updating info dtill takes me back to update or add another account! Please cleanup your act!!!!
Paying your bill online has not been down for since that time. I am not sure why you cannot pay your bill online.
Is your online profile at the account level? Here is a blog that gives you the steps on how to check and correct it.