Certified or Registered Mail
Credit cards sent in a package that is insured and sent via registered or certified mail is a safe way to ensure that the credit cards reach their intended destination. This is not to say that registered or certified doesn't still get lost in the mail, but this allows you to trace the package and see where it last checked in en route to its final international destination. Whomever the credit cards are being sent to, the receiver must sign for the package when it arrives to ensure they are the person who is supposed to be receiving it.
Hide it in a Package
Sending a package of something else, like a children's jacket in a padded box, is another safe way to send a credit card, as someone stealing international mail is probably not going to risk stealing a children's jacket when they are looking for checks or credit cards, which is quick, easy money. Simply slip the credit card or credit cards into the pocket of the jacket and fold the jacket over this pocket before you put it into the box to mail it. This way, even if someone can see inside the box through a scanner, they won't know that a credit card is in the pocket of the coat and will be less likely to tamper with or steal the package.
Send it with Someone
Of course, the most expensive but safest way is to send the credit cards overseas with someone you trust. This person is least likely to be robbed or lose the credit cards along the way, as long as you trust them. Finding someone you can trust to carry the credit cards with them to where they need to be delivered is the first step. If you find someone who is already heading to that destination who you feel comfortable transporting the credit cards for you, even better, as it will save you money in having to fly them overseas just to deliver some credit cards safely.