Autopay is great for fixed things (car loans, student loans) but for variable payments I like to pay it on my own because I'm always worried they will jack it up somehow. Comcast and VZW in particular at bad at billing. Save your comments, they still provide the best product even though the customer service and billing are sub-par.
I like to set those up through my
Giant Monster Mega Bank's online banking interface. If the creditor is sophisticated enough to be compatible*, they can send e-bills so that the bank knows how much this period's variable payment is, then you can choose from several options of how much to pay: the total account balance, the amount due (i.e., the minimum payment in the case of a loan or credit card-type account), a fixed amount, or the amount due
up to a maximum you set (which is the option that's helpful for asshats like Comcast). You can also set it such that it will autopay if the amount billed is as expected, but email you if the bill is unexpectedly large.
(* My water/sewer service is provided directly by the city instead of a privatized utility, and is the only creditor I have that is both variable and
not compatible with e-bills. I can set them up as a "pay to" account, but there's no way for them to tell the bank how much I owe so I have to manually receive my paper bill and type the amount in. It also means that's the only bill I occasionally forget to pay and get charged late fees for. It's infuriating!)