If you follow the link on the email and click on 'buy' enterprise you should read:
Free for the first year
Then €159.00 EUR for each year.
I have sold a one-time fee service based on the PRO version to my customer: I told him there is no annual fee, so I cannot upgrade: I should pay it for my customer next years, without having an income for this!
However I hope GTranslate team may understand this: it is difficult for us to upgrade, because we don't know how to sell it to the costumers, who paid the service fee one-time.