With this form, I've spent a couple of days building it, so I need a donation of at least $30 / £20. Once I have your donation, I'll email you the script. You can add the script to your page or link to it.
View the source of this page for all comments.
I've put comments in the script and html so that you can see where needs changing for specific products or options.
With all products and options within this form, you need to add either... onchange="sumFields();" oninput="sumFields();" or ... onclick="sumFields();" onkeypress="sumFields();" for all variables.
The onchange command is for either a dropdown list or a text box, the onclick is for a checkbox.