The fee varies on the amount your bill is. However, Self Serve is working fine for me. If you are having issues, try a different web browser or delete the cookies and cache from your current web browser and try again!
I believe late payment fee is 2%(if they didn't change).
My self serve is working fine too.
Try Mathieu's suggestion and if it didn't work you can call Koodo *611 from your phone and make one-time cc payments for this time.