StaxelKey
From wiki
A StaxelKey is a game code that is needed to turn your staxel account into a premium account.
When buying Staxel through humblebundle.com or http://blog.playstaxel.com/store the staxel key is on the redeem page above the link to the installer, you can go back to the redeem page through the humblebundle.com email or through your humblebundle.com account.