prepay

Cash is illegal to send by post, however, which complicates matters.  It's also generally not a good idea (tm).

It most certainly is not illegal to send cash by post.  It might be inadvisable, but most certainly is not illegal. 

I'll echo Charlie's statement, please do NOT send cash by postal mail, regardless of whether or not it is legal.

James has stated he does not want to use any sort of electronic payment system at this time.

Its not illegal to mail cash.

It says on every piece of paper currency (under the Federal Reserve seal) "This note is legal tender for all debts, public and private"
I have a story about a $1000 note in 1980...