Owner Julie Baggott did not spot her pet Cupcake when she packed up a box of DVDs and sent it off to a customer.
Eight days later, the female cat arrived in Worthing, West Sussex, after somehow surviving the nearly 420-km journey from Falmouth in Cornwall.
The customer was understandably "somewhat startled", vets said, when the box of DVDs was opened and Cupcake jumped out.
After being contacted, an animal welfare charity collected the cat and took it to a veterinary hospital.
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"She's done really, really well, she's responded to fluids really well. Luckily she was micro-chipped."
After scanning the cat and discovering the microchip, they found Baggott's details and got in touch.
The owner had been distraught at the loss of her cat. She and her family had put up posters and searched around Falmouth for days.
"When I realised she was missing, two weeks ago, it was the most horrible, scary feeling," she told BBC.
"I feel terrible about what's happened... I put everything in the box and I sealed it straight away, so I don't know how she managed to get in there.
Cupcake required several days of treatment for dehydration but has made a full recovery since being collected by Baggott and taken back to Cornwall.
Although the Royal Mail does accept some live insects, bees and earthworms in the post, it does not accept live animals.
In 2006, a Cambridge University student was fined 750 pounds and banned from keeping animals for 10 years after he and a friend posted a hamster as a prank.
The hamster gnawed through the envelope and was rescued by a postman, who decided to name him First Class.