It is completely safe using your credit card at 21winners.

Intersafe Global, which provides e-cash and credit card processing services for 21winners, is one of the leading e-cash providers, with over US $60M of on-line transactions processed. 

Intersafe Global utilizes sophisticated RSA public/private key encryption technology to ensure that sensitive data is transferred securely over the Internet. All credit card details are stored on a secure server, which is protected by the latest firewall system.
 
This means that using your credit card at 21winners is far safer than using it to order a pizza or at your local supermarket.