users enter the shipping fee after they phoned the company

[vidadio]

how do I make user can enter the shipping fee after they phoned the company and gain the shipping fee amount?

I gave a certain shipping amount if the product is going to send to UK or EU countries.

I set up a input box for users to enter a shipping amount if the product is going to send to other countries but not UK and EU countries. 

I'm updating flex shipping module and the field I use is shipping_rate_id.

Could anyone have ideas? thanks in advance.