I believe that it is against most online sites, and I think that the mailing system here in America is totally against sending cold hard cash in the mail. Usually, for retail sites, you need to have it in the form of a check or money order. Also, of course, credit/debit cards are used too.
I'm sure someone from T11 can clear this up for you more.
I would not recommend sending cash. Very easy to steal and very hard to trace once it has been stolen. I would suggest purchasing a money order or getting a Cashier's check form the bank. Neither are expensive (a couple of dollars each) and they can be traced and replaced if lost or stolen (please confirm that with your place of purchase).