I didn't ever type in the URL from the paper bill (BTW, my only complaint is that I wish there was a way to shut off dead-tree billing). I just went to the assessing page, clicked through to the bill payment thing, and typed in my checking account number.

I have other online billers with much worse sites. This one isn't bad at all.

I paid mine in about 5 minutes, for three separate parcels. (There is a link directly to payments on the cithyofboston.gov homepage.) I do wish you could pay with a credit card & the navigation could be better (e.g. a link to pay an additional bill instead of starting over) but it wasn't too bad.