questions about paying for shipping

Issue Summary

Since I didnt get any responce from commenting on another issue I have decided to open a new thread.

My question is about the logic behind using the userpoints payment method, currently if I choose to pay with the userpoints payment method instead of a credit card it only checks if I have enough userpoints for the price of the product but not including shipping.. then when I checkout it does charge for shipping also but if the user had just enough for the product only he ends up with a negative balance....

So what I would like to know if it was designed to work this way?, and if the answer is yes then what is the point of the payment method when you could simply be using the discount module? except with better results since you would be able to pay for the shipping with the credit card..

Please answer these question if you can, this is really buggging me since I wanna be able to offer the users to pay for the whole thing with userpoints!

Regards,
Gilli