Profit margin

[Cap'n Jack]

Nope you are wrong, they are not a part of the mag. If you don't believe this try buying a mag with them installed

It seems Aircraft Spruce sells them.
See: https://www.aircraftspruce.com/cata...MIjLWyvp7r4QIVA6rsCh32XACsEAQYASABEgJk8fD_BwE

Note: Accessories include the TCM/Bendix IMPULSE Couplings

This one lists rotation, and if it has an impulse with the prices (application guide tab):
https://www.aircraftspruce.com/cata...MIjLWyvp7r4QIVA6rsCh32XACsEAQYAyABEgIibPD_BwE

I believe you are carried away with my scenario of how a job can get more involved than anticipated. Nit pick it for all you are worth, It won't make it go away. stuff happens

Actually, the topic is very germane to your question. Proper and clear communication is important when a job becomes "more involved than anticipated" especially when the customer doesn't have your level of knowledge, and reads your task list differently than you intended.
As you said, "stuff happens", and dealing with the "stuff" is easier when both parties understand the extent of the work and the associated costs.