Should the airport stamp be filled out and put in an envelope and mailed ?

Where can I return cash?

Right at the airport refund counter

There is a mailbox next to the tax refund.

Can get cash back

If you want to get your cash back, you don't need to mail anything, just handle it in minutes. Opposite the entrance to Prague airport, there are two companies. One with a blue sign and one with a yellow sign. First queue at the customs next to them to get your stamp, and then queue at the blue or yellow sign corresponding to you.