It is illegal to send negotiable instruments such as bank cards, credit cards, and the like through the international mail (this requirement also requires that cheques be not negotiable and prohibits the mailing of cash). Many convertible items (e.g. bonds, shares, deeds) are also prohibited.
As far as I know the fine is up to $5,000 (Postal Services Act).
This is a UPU requirement so will be illegal in any member country. I would have thought that the reason why this is so would be obvious, but appears to not be so to some.