I've managed to throw together the from below. It has a two stage calculating method. It displays the amount (subtotals) for a quantity of items at a fixed amount - there are several items. You just type in the number that you want. The amounts (subtotals) are then added together via a button click to produce the overall total.
I'm aware that the script is quite basic, but it works for what I want it to do.
There are two aspects that I'd like to improve:
1) when a letter is typed the resulting value is NAN - I'd like to change this to a blank output.
2) I'd like to dispense with the 'add sub totals' button. I would like the total to add based on the calculations taking place further up the form. I've tried to integrate scripts that do this but I haven't had any luck.
Any help/tips would be greatly appreciated.