Depends how complex you want to get. For a simple form (one or two products) you could use some simple javascript. For something complex you might be better off checking out shopping cart software.
You may use Microsoft Excel to put the calculation part of the form in if you know how to do calculation in MS Excel. You can then change the excel page into an html page and put the code into your site. These sites may help:
http://www.createforms.com/ I am not sure of this one never used it before.
http://www.thepcmanwebsite.com/form_mail.shtml This is pretty reputable site
