Is it working with altcoins? Or is it just only for BTC?
It works only with Bitcoin.
You're faq states something else:
http://doublebtc.in/faq9. Which cryptocurrencies do you accept?
We currently accept over 4 different types of cryptocurrency: Bitcoin, Dogecoin, Litecoin and Paycoin.
This is a demo version of the script, the script is constantly being updated and will eventually be added to another Dogecoin, Litecoin and Paycoin.