No, Zarathushtra does not promise that doing good will be returned with good another day.
What Zarathushtra is saying that we must understand that we BECOME the thoughts we think, the words we speak, the actions we undertake. We are BOUND to identify with how we relate to and interact with the world.
Most things that happen to us are out of our control. Zarathushtra was well aware of that. But he was preoccupied with our ETHICAL self-identity, our view on our ourselves as ETHICAL rather than moral creatures. Zarathushtra was even the inventor of ethics (according to Nietzsche in "Ecce Homo").
This is hardly primitive stuff but rather a philosophy of ethics as advanced as the best in the west (such as Spinoza, Nietzsche, Heidegger and the other existentialists).
2008/8/5 Special Kain
I'm most definitely a newbie and not too familiar with Mazdayasna, so please bear this in mind when reading my answer now.
But when I read such passages, I often have the impression that Zarathushtra promoted a rather "primitive" feedback loop that I don't agree with: Do good, and good will return one day.