What is JotForm?
JotForm is a free online form builder which helps you create online forms without writing a single line of code. No sign-up required.
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I took a look at your jotform and I saw that the calculations did not seem to be working properly. However I have noticed that you are using custom properties.
Now they work differently from the quantity, they are seen as different options that the user can choose from while paying the same sum. For example a picture frame in red and blue color.
Now if you want to change that to have different prices, you will need to edit these options and turn on special pricing. You can read on that here What is Special Pricing?
Here you can see a clone that has a quantity option selected: http://form.jotformpro.com/form/42804356768969
You can clone it to your account by following these steps: How to Clone an Existing Form from a URL.
I would suggest using the quantity option as it is quick to set and easy to change without requiring to make calculations by hand for special pricing.
If you have any further questions please let us know.