Previously, I shared this message from eBay about rate increases with UPS and FedEx. While eBay has not yet updated their announcements with USPS price increase details, I found this link to help you plan.
Below is the flat rate pricing chart for Express Mail and Priority Mail that will go into effect as of January 22, 2012 (if you click the image, it will take you directly to the USPS page with new pricing information).