At the post office

Paul: Good morning.
Clerk: Good morning, sir.
Paul: I'd like to send this letter to London.
Clerk: Sure. Would you like to send it as a registered letter?
Paul: OK. What is the fare to London?
Clerk: One pound fifty.
Paul: OK. I have to send a package to this same address next week. How much would that be?
Clerk: Well, that depends on the weight of the package, of course.
Paul: It may be around five kilos.
Clerk: Then it would cost about ten pounds.
Paul: And if it were only one kilo?
Clerk: If it were one kilo, it would cost two pounds.
Paul: Thank you.
Clerk: You're welcome, sir.