Not buying it.
1. If they were expected for show prep, it would have been easier to send them electronically in the first place. And they still could have sent them electronically in seconds once they learned that the package had been damaged. Zero prep time lost. They still could have run the story, but they didn't. That tells me that, at best, they're colossal idiots and sent the only copy of the documents.
2. How the heck would a random UPS employee know anything about the contents of an express envelope from one Fox studio to another? If an employee truly did tamper with it, it wasn't in an intentional effort to suppress this story.
ETA: Also, it kind of is about the effectiveness of the "suppression". Regardless of reason, the story didn't air. It was suppressed. That means that either they sent the only copy of the documents (which I find extremely difficult to believe) or the story had no legs to begin with and was suppressed internally - and Tucker Carlson is now using it to manufacture outrage.