When I send cash gifts in the post I always use a manilla envelope, never had a problem. The PO tried to strong arm me into using a money transfer service but it was bloody extortionate to send €20.
While I will send birthday money in the post in this country I certainly would not risk it going abroad, plus you would need to get the currency of whichever country it's going to. A pre-paid card could pose the same problem. Whichever means you choose there will be costs involved in converting the currency.
Ideally I'd put something in a card and send it in the post as its a surprise gift. I'd have to ask for details if I transfer to either their account or to Paypal.
So leaving aside the cost, what are my options for a couple of hundred quid as a surprise? I'll head off and research the cash card option, sounds useful.