
Just leave it 2 more days and after that I'd just assume it's been lost or stolen. Thousands (if not millions) of birthday-cards get stolen every year, it's easy enough to spot birthday-cards and they often contain cash meaning they're often stolen in hope they contain cash.
In future try putting the card envelope inside a business envelope so then it hides what it is, or you could just put sellotape across the seal of the envelope, however this doesn't necessarily mean it won't be stolen.
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