i dont think your supposed to send cash in the mail or by courier im not sure what checks they do to detect it,but when i sent my wife a card and other stuff i put £100 in a envelope inside the card and she got it ok.
she went to the post office to collect it it was all opened and checked but they never opened the card!

what reason did they give for taking your money?

when i was there last i took £4000 with me and was never checked!