extending your sales team

The conversation usually begins with an email to an engineer.  It takes shape something like this: 

[Customer] "I need to add this additional feature to our current project.

[Engineer ] bluntly  "It's too late to add anything to the project scope." 

[Customer] "I understand that it is a little late in the game, but our client says the feature is critical to their application."

[Engineer] "Why didn't they tell you that before we scoped the project? We'll never get done at this rate!!"