To reduce the risk of compromising sensitive data hosted on your server, you might want to implement special security measures that comply with the Payment Card Industry Data Security Standard (PCI DSS). The standard is intended to help organizations protect customer account data and enhance system security.
Parallels has released a comprehensive PCI Compliance guide for the Plesk hosting panel for both Windows and Linux. A full PDF copy is available here and an online version is available here.
I recommend using Parallels guide as it is maintained inline with industry standards.