Ok, so putting up tips here instead :P

I personally havn't read c++ yet, so do not exactly know what you can do and how easy it is, but I assume that it's a lot like c#.

Make it possible for user to choose creature products from a list "checkboxes or something" that wants to be calculated. The user will then get to a new window form where he/she enters the amount of each creature product. Next step will calculate the value and show the amount of creature products that should be sold to each NPC