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One option is under "My Account". Click Billing when you log in and it'll give you a big green button that says Pay Bill.
A variety of options to pay your bill are on this page, linked from 'Account & Billing' at the bottom of this page.
There are options for credit card, bank transfer, cheque or money order available to you.
I also have this issue. I've just received my first bill. Before paying it, I set up pre-authorized payments (PAP). I think I saw a button for manual payment before I set up the PAP, but I couldn't see it afterwards. The dialogue said the PAP would take a full billing cycle to come into effect. I then cancelled the PAP in hopes that the manual payment option would reappear, but it didn't. The dialogue also said the PAP cancellation would take a full billing cycle to come into effect... so now I can't pay my bill ?