Is it permissible to register on the website more than once to get the discount, knowing that registration only requires an email?

There is no harm in that, unless the site stipulates that each user registers with only one account, in which case the condition must be fulfilled; because Allah Almighty says: "O you who have believed, fulfill [all] contracts," and the Prophet, peace and blessings be upon him, said: "Muslims are bound by their conditions."