It was possible to get "pro" version credits with coupons from very different places, for example Humble Bundle book bundles. Sometimes they gave them themselves. I guess you were also able to buy credits directly, but I am not sure about that.

Well, anyway when the credits ended, your service did not downgrade but they put your negative credit automatically, and if you don't pay it, you were never able to use your account until you pay this negative part.

There was no mention in anywhere that this would happen, and also all the messages were quite threatening.