Your credit union (you do use one, don’t you? :-)) or bank should have a online bill payment service that you can use to pay any vendor. That’s one way to avoid using the vendor’s site. Also, certain credit cards (e.g: American Express, Chase One Visa) will let you setup AutoPay to pay the minimum balance or the entire amount due, automatically, from your bank account. Additionally, services like Manilla (http://manilla.com/) offer bill consolidation and payment. How much luck you will have with the latter I wouldn’t know.
That deals with services and options for bill payment. Now turning to your issue with passwords:
You can use software like LastPass (http://lastpass.com/), KeePass or 1Password on your computer to have them remember your various site passwords for you and enter them into your browser when you visit those sites.
You can also use simple algorithms to generate a unique password per site in a manner that they are different per site but not requiring you to remember each one. I recommend a difficult to guess password string modified with a few letters from the particular site you are dealing with.
Finally, I use a standard secret (password) for all those “Secure Questions” irrespective of what the question is. So, for instance, if Site X asks me to answer two questions, I choose any two (What hospital where you born in? What’s your mother’s left foot length?) and supply as an answer the two passwords I always use. After all they don’t check that the answer makes sense.
HTH,
—ravi