It does work as you can see here on this quick and basic test site.
https://givemeawhisky.wixsite.com/mysite-4
The three dropdowns with a value of 100 each will add upto 300 in the sum field.
However, with your payment setup in the backend, you are only allowing for the single product so as said in the code above, you will need to change the charge to say 300.00 for price and amount for it to work and show 300.
Plus, make sure that you have made sure that your site is setup to accept payments.