I am not a gunsmith, but I have made many quotations, and ensured the work was completed as close to the promised time as possible.
We charged what the parts costs us. If the part has not been made in 30 years, yes, it will cost more than an in stock part for the current machine models. We would need to make it, or have a machine shop make it, harden to specifications, and so on.
Parts are paid for upfront. Labor, travel expenses are on the quotation, 60 day price stable, and when the purchase order came through, the service date was set. The PO was not charged until the job was complete.
If I was doing a programming job, there was a $1,500.00 fee upfront. Before the job began, the customer would provide what they wanted. I would write what I understood they wanted. If so and correct, they signed and work began.
This helped us stay on track, and not upset too many customers.
Squeekie wheels did get some preference. But, "The boy that cries wolf is soon not listened to".
The place in line begins when you provide a purchase order number.