Sounds like the resin wasn't quite set when the cardboard was slid on for shipping. Either that....OR, it's possible that the resin softens up a bit in the extreme heat of the summer, in the back of a USPS truck...and then the cardboard adheres to it a bit when it re-hardens. Other thoughts?

JYD #121
The chief cause of unhappiness and failure is trading what you want most in life for what you want at the moment.