Interesting concept! I like how the "bonus" system encourages you to keep parts in stock, though it's hard to plan ahead when you have no idea what parts are going to be required next.
It took me a second to realize I needed to click the "end week" button because the placement makes it look like just another tab. If you made it larger and placed it in a different location it would pop out more.
I think the whole interface could be made into one screen (Quick mockup at bottom). The inventory and order system would be combined, and when you order something up the number would change color to reflect that you've ordered more. You could also have a number below the credit amount to show how much you've spent that week.
