If you work in a field where things need to be delivered by date certain, with zero defects, with high consequences if you make a mistake—then you need to charge a premium for exposing yourself to emergencies.
It doesn’t matter what something in a non-emergency situation costs. If someone wants the standard version, let them buy that.
The buyer is offered to pull it off the shelf, see if you like it. If it doesn’t satisfy you, take a different one.
Emergencies (or even the risk of emergencies) cost extra. Yelling at us costs extra. Panic costs extra.
Your entire organization (and your entire day) revolves around preventing the emergency or recovering from it when it occurs.
The reason custom work costs more is no longer a matter of production efficiency. Computers are happy to customize things.
Big companies that serve other big companies spend at least 80% of their overhead on being ready (or dealing with) meetings and emergencies.
The reason to charge more is all about ensurance, insurance and emotional wear and tear.
If that’s the sort of work you want to do, charge appropriately.