Shouldn't need any javascript, this is doable with calculation fields.

If you name the line item price fields as "ItemPrice.1", "ItemPrice.2"...etc. then you can create a field to calculate the sum of "ItemPrice"

Create a field for the gratuity and the sales tax (set the Default values to ".2" and ".065" and make them uneditable) and you can then use them in the calculations (sums and products) for the other 'Totals" fields you need.