Well Flat Rate is a whole different story. I assume it was a Flat Rate Priority Mail. Was it unusually bulky with a lot of tape? Here are a couple of instances that are similar...
https://answers.yahoo.com/question/index...345AAr7JfN
https://www.reddit.com/r/Flipping/commen..._being_to/
https://answers.yahoo.com/question/index...345AAr7JfN
https://www.reddit.com/r/Flipping/commen..._being_to/