If i were serving you, i’d check our margins, check if 20% is workable (i’d doubt it unless the shop was keen to shift). But offer to fit (including cutting hoses and bleeding) for 10-15% off dependant on what margin we’re looking at.
Plus i’d make the argument that if you have any problems, we’re here to sort it. Any warranty has (99% of the time) to go through a shop anyway.
Does that seem reasonable?
Don’t go in all guns blazing, nothing wrong with enquiring if your LBS can match. It’s also silly to turn away a customer who has cash there and then and is wanting to buy. As david dickinson says “cash is king”.