Posting cash is entirely legal and above board!  As anyone who travels
to and from America will be well aware of, the U.S. Customs form you
fill in on arrival says "There is no limit on the amount of money that
can be taken out of or brought into the U.S." -- but you have to
declare the amount if carrying more than $10,000.  The era of absurd
currency controls is long gone.

The general reason that sending of cash (and indeed, "any valuable
articles," to quote the U.S. Postal Service web site) is deprecated by
authorities is that untraceable easy pickings encourage sticky-fingered
employees!  That's why cash is "restricted" (not "prohibited") and
should be sent via registered post, which is handled entirely separately
from other mail.  There is a $7.50 fee for registering.  With low sums
individuals may question the value they get from the $7.50 and choose
to risk it, but this is certainly not a Federal offence!

To quote directly from the USPS web site's country conditions about
mailing to the UK:

  "Coins; banknotes; currency notes (paper money); securities payable
  to bearer; traveler's checks; manufactured and unmanufactured
  platinum, gold, silver; precious stones; jewelry; and other valuable
  articles, may only be sent in registered letter-post shipments or
  insured parcel post."

Cash is therefore legal (up to $10,000 anyway) in registered
international letter post.  However, registration is not available for
some of the cheaper postal forms such as M-Bag and Global Priority
(which is what was quoted in yesterday's MMD).  You'd have be sending
rather large wads of notes to need anything beyond letter post!

A point not made so far is that Traveller's cheques count as cash in
all these regulations, especially once they have been double-signed.
Posting them in lieu of cash is simply a nice way of handing money to
the cheque vendor.

Julian Dyer