This is a classic scam letter - he doesn't want the cat at all - he wants your money. The scam is he pays the adoption fee and shipping cost by cashiers check and overpays you, and instructs you to send a third party (also in on the scam) the overpayment. The check bounces, and you're out the overpayment.

This type of scam has been going on for years, usually for big ticket items like cars, boats, motorcycles...etc. Any way to make a buck, I guess.