Adding Calculations On a PDF Form

I'm trying to create a fillable PDF purchase order form to replace an Excel document. The way I need the PDF form to function is simple. The user enters quantity of a product they want to order into a row for that product. The quantity is multiplied by that product's cost. The total cost for that orders gets into into a cost column for that row. (product A: order quantity x cost = total cost)

I need two separate columns at the bottom of the form. One column will total the order quantities and the other column will total the costs column. It seemed like a simple thing to do but I've run into a LOT of problems.   

There are three pages to this form. When I enter in a quantity in page 1, it shows the correct total for that order but the totals at the end of the form double the total costs. Ex. If I want 2 of a product that costs $10.00 it correctly shows up as $20.00 in its "totals" column; however, the totals at the end of the form are doubled. Pages 2 and 3 don't have that problem.

2.) The form functions somewhat slowly. I check for updates. 

I know Adobe uses javascript to create fillable forms. I thought I had a fundimental understanding of how it works. (I reviewed several articles and watched several videos.) Maybe I don't have the thorough understanding and command on how to create a fillable form. By the way I have the latest version of Adobe Acrobat.

Can someone help?