Some advice specifically for custom orders:
You should be able to get a much better discount unless the car has a limited allocation and there is a shortage (as in, there’s an actual waitlist to order).
Otherwise, use this to calculate invoice: https://www.bimmerfest.com/news/917430/calculating-bmw-invoice-pricing-for-my-2017-models/
Then subtract another 4% from that.
That should be your expected sale price before any fees, taxes, or incentives. That will give you a very achievable discount on a custom order.
I would then email the dealer and say something like, “I want $$$ as a discount. I know you will still profit about $650 plus doc fee on that sale. I can easily get that price by contacting dealers on enthusiast websites and doing a performance center delivery, but I would prefer to keep my business local.”
It takes literally 5 minutes to put in a custom order and usually as soon as they know they are competing with the entire country, they’re a bit more willing on the discounts.
If they don’t play ball. Walk away and actually do it.
If you have the time, I recommend doing the performance center delivery anyway on a custom order. It’s free (other than the time and airplane ticket) and a lot of fun and educational.