You want photos that don't look like a frantic school picture day. The best way to get that is by picking a weekday morning when the light is soft and the locations aren't swamped with weekend crowds.
Booking these sessions takes a bit of planning, but it's worth it for the result. Here is how to keep the cost manageable while you get the portraits you actually want to hang on the wall.
That 0% window is a deadline, not a gift. If you haven't paid off the $450 by the end of the 18 months, you will start owing interest on whatever is left over.