In the "Payment settings" of your Donorhut control panel you can require every donor provide their zip / postal code for credit card verification purposes. If enabled the provided zip / postal code will be compared to the credit cards zip / postal code on file at their bank. This provides and added level of fraud protection and helps you avoid unnecessary charge backs with credit card providers.
Be aware that some credit card providers may not have a postal code on file for the credit card, which may result in the donation being rejected.