Some players may have been charged full price when upgrading their existing habitats to premium habitats during the 50% off sale.
We pushed a fix to correct future purchases so they receive the 50% off discount. Also, we have credited the difference in Gold back to players that didn't receive the 50% discount when they upgraded their habitats. You may need to restart the game for the changes to be reflected.
Thank you everyone for letting us know about this issue we can investigate and fix it for everyone!