Kant's argument from gratitude

Kant famously argued that we had a duty to believe in God, as this was necessary for us to fulfill the duty to give thanks for the universe. My understanding is that Kant thought that this was not an argument for the existence of God, but only an argument for the duty to believe in God. But surely it becomes an argument for the existence of God (or at least an agent who caused the universe) when one adds the very plausible premise:

• One only has the duty to give thanks for a product of agency.

Perhaps, though, Kant would want to subjectivize this premise into:

• One only has the duty to give thanks for what one believes to be a product of agency.

But if one says this, then the atheist doesn't have a duty to give thanks for the existence of the universe, and Kant's argument fails. So it seems that either Kant's argument fails (at least in the case of atheists--maybe you could argue that if you believe in God then you have a duty to believe in God, which is an interesting result, but I don't think it's what Kant was trying to argue for) or Kant is wrong that we can't argue for the existence of God or both.

But I am no Kant scholar.