$40 sounds unusually low for a radiator flush. Where I live it seems to cost at least $90, and usually more.
I’m confused about coolant changes and I can’t get any straight info. I’d like to possibly change mine myself, but every source I go to has a different method. Some sound easy and others sound difficult. Some just change part of the fluid, while others flush everything out with a garden hose (as opposed to the specialized flush machines they use at shops.) And when I go to shops, some say drain & fill is okay, while others say that you’re always supposed to do a flush.