That isn't mostly rust buildup. That is the edge of the rotor that your pads don't touch. If it feels raised then that just shows you how worn your rotor is.
You do not always need to replace rotors with pads, but it looks like you'll be replacing them next time at the latest.
I once had a rotor split while driving home from 6 hours away. When it cracked I was riving 60mph on a State Road, the crack seized the rotor when it caught on my pads. My rear tire locked instantaneously on a double lane, driving 60mph. Sliding through traffic isn't fun, and neither is changing rotors on the side of the road.
If you're concerned about money, call your dealer and tell them that you want them to use lifetime warranty parts. I they don't have any lifetime warranty rotors, tell them you will provide them with some. You can get them at any auto parts store and they don't cost much more. Autozone charges me $2 extra per lifetime warrantied part.
Then when it naturally wears out, they'll replace it for free.
Worst case, changing rotors is literally one of the easiest tasks to do on a car, it's definitely one of the simplest. You can do it yourself if you want to save some money, but do it right after getting the pads or you'll sacrifice some of the pads life as well.