help with series of custom calculations on PDF order form

I have a PDF order form in which there are a few calculations happening at the same time and I've run into a road block about how to handle it using Javascript. I've attached a picture of the part of the form that I'm hoping to have calculated. I was able to calculate the "Pre-Total" field named "Sub_Total". But then the next field "Min_Handling" would only be calculated to add $2.50 if the Sub_Total field is less than $15. Otherwise, the value should be zero. The next field for UPS Shipping would be based upon the amount in the Pre-Total field and would be three different flat rates based upon the shipping charge rates displayed. The "SUBTOTAL" field would be the values of "Sub_Total" + "Min_Handling" + "UPS Shipping". The next field "PA_Sales_Tax" would be calculated at 6% of the SUBTOTAL field ONLY if the shipping or billing "State" field is PA (Pennsylvania), otherwise the value would be zero. Then the "ORDER_TOTAL" field would be the sum of all the fields from Pre-Total down. There is a lot happening here with calculations and I have myself confused. Any help would be appreciated. Thanks!