1)Maybe its my screen resolution (1280*1024), but the "Pay now" box overlaps the content of invoice sheet.
2)Where the hell is login? Not where I would expect it, somewhere at the top. its at the left side... In the middle?
3)Why is there a "tip" box that is titled "How?" What does that even mean?
4)What does the open button do in the bottom left corner? Er... Ok.. It saves my invoice...
5)Everything is just kind of floating around, no defining lines or colors. Makes it very hard to decide what is part of the web app and what is not.
This needs a major amount of UI changes to even make sense.