So far no reports of credit cards/customer data from online shipping, but wouldn't surprise me if that changes soon. May be a good idea to keep an eye on the accounts if you have used Click+Ship to send anything out recently.
http://www.reuters.com/article/2014/11/10/us-cybersecurity-usps-idUSKCN0IU1P420141110
http://www.reuters.com/article/2014/11/10/us-cybersecurity-usps-idUSKCN0IU1P420141110