> My contention was/is: if you were checking your banking accounts routinely, you'd know that specific bill wasn't getting paid.

Unless they have been being charged the whole time. Given that their bill doesn't seem to have grown relative to the others they just paid, I'm guessing they were being charged.

Regardless, the city offered a means to a payment, the resident accepted and completed it. Offer then acceptance. This is all that's needed for most contracts.