Adobe Acrobat forms script to calculate shipping

[tdxpress]

Hello,
I need help creating a script to use in an Acrobat PDF fillable form. It's a simple form to order one particular item. However, the buyer may purchase up to 9 of this item on this order form. I would like the form to automatically calculate the shipping to be $10 for 1 ordered, an additional $5.00 for 2-8 qty, and free shipping for a quantity of 9 ordered. Since the form has other calculations based on qty (ex. price x qty = subtotal) I need the value of qty to be two different things. I need it to be the actual qty number for the price and I also need it to be a dollar value for the shipping calculation (i.e., 1=10, 2=15, 3=20, etc.)
Any help is greatly appreciated!