Help me with my conspiracy theory!

Just got home and received a small package through the postbox, royal mail tracked 24.

The package was addressed to "The Occupier" at my address. My thoughts went to "what the hell would be sent on a package this size, addressed to the Occupier, and on 24h tracked mail?". Mind, the package was not much larger than a bank card, same general rectangular shape, and quite thin.

A couple of options for what it could be went through my mind, none of them very pleasant. To contradict the most rational thing I could have done, of course I decided to open it.. It was a birthday card with a "Happy birthday" message on the front, and nothing else written on the inside. I am pretty sure today is not the birthday date of neither of us that live at this address.

So, how long do I have until the anthrax kicks in??

I'm counting on this community to help me solve this mystery!