Is the homeowner claiming that an install you did is undersized, and asking you to give them something, or is the homeowner claiming that their existing service is undersized and asking to buy something from you?
If the former, show then a proper load calculation, and explain that while the panel would be overloaded if all of the connected breakers were being used at full capacity all the time, in a properly designed system not everything runs at full blast all the time.
If the latter, state that your charge for a proper evaluation of their electrical system, including a complete load calculation is $XX, explain that without full analysis their system sounds fine, and ask if they've noticed any problems that they think require a professional evaluation.
-Jon