That'll drop the car about 3/4".
Each spring pad "lug" adds 5 mm to the spring pad and 10 mm to your ride height.
HD's may have risen your ride height a little. If you want to REALLY lower it, then shorter springs & Bilstein Sport shocks are necessary.
Changing the ride height changes alignment. MB says at 20 mm change, get an alignment. That's what I'd do.
2-2.5 hours to do four springs is a great price. Add $100 to check/adjust the alignment. Their machine should have a levelmeter to read the ride height and use the best settings within the allow range.