How do you close  down discussion with a client when he gives one line requirment stating I need a solution for my sales representatives who could buy promotional items online and place orders for them.

He needs a fixed cost for same and is not ready for further discussion.

Here is what I thought of my approach

Option 1

Provide him WBS with the various components to be developed and asuumptions and provide him fixed bid cost with a ROM estimate. But again this is Fixed bid project I dont want to give him a range so I am thinking of going ahead with PERT estimate. High risk involved but cant help

Option 2

Run a Agile project in series such as Scrum and provide a monthly run cost. All though this is not helpfull for him becuase he looks forward to convience his stakehloders the cost that I gave and get approval for budjet.

Client is very strict and not ready to adjust one the fixed bid cost is proposed. Not even a formal Change request board is encouraged. he expects us to provide solution in that fixed price

Any suggestions how to handle? I thought of above 2 ways