The trigger "Update order status on full payment" doesn't fires up and why does the balance condition is set to <=0 ?

I'm misunderstanding something with this module, especially with Conditional Actions...

First of all I've created Sandbox account. Added Seller and Buyers. After that I've installed the Ubercart module and set everything like it said in the documentation and like on the images )... added few test products... Than I made a few test purchases on web-site... and everything is great except one thing! I need to sell files. But the trigger that "Renew purchased files" is called only when the status of an order is completed. And it should be that way and no other. But the order status doesn't updates for some reason. After the purchase is made the order status is always set to pending. So, to make file downloadable for customer, I should change the order status to "Complete" manually. This is not what I'm expecting from this system!

So. I tried to experiment with the triggers of Conditional Actions.

The first step.
The first trigger is "A payment gets entered for an order". And there is only one action named "Update order status on full payment". I've deleted all the conditions and added in the actions tab new action to comment an order when this trigger is fired up. After that I've made a checkout, completed the payment procedure and viewed new created order, but no comments where added to the order. That's why it turns out that this trigger is useless, at least for me.

The second step.
The next trigger is "Customer completes checkout". I've done the same actions with "Update order status upon checkout completion with full payment" action and it turns out that this trigger WORKS. The conclusion: there was some wrong Conditions. The main condition is "Check order balance". It was set to "Balance is less than or equal to $0.00.". But why??? After the checkout is completed the order's balance is equal to the price of an item. That's why I've changed "Check order balance" condition to "Balance is greater than $0.00." and wow! It works! And after that all other triggers started to work, except the first one of course.

So, my first question is why does the balance condition is set to zero by default?
The second question is what's for the trigger "A payment gets entered for an order" if it does not fire's up at all?
Also, I've noticed that on my web-site the order status is changed at least to pending only if the customer clicks on "Return to " button after he payed for an order. Otherwise the status of an order is "In Checkout". Doesn't this status should be changed when the customer has payed the money?

I would be grateful if someone could point me out and give me some clue. I know that there is many posts that are similar to my, but didn't find the answers on my questions there.
Thanks!