I "auto pay" all of my billa through my bank (Fidelity) and they have an option to automatically pay up to $x amount but just email me if the bill is higher. I'm pretty sure it was intended for bills that aren't quite the same each month, but it would also work here.

Most billers have the option to send the bill directly to my bank, for the ones that don't, then it has to be a fixed amount each month for Fidelity to auto-pay it.