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I'm sorry for the trouble caused to you.
Please let us know with which form you are having this issue with. I checked your last edited form "Marine Basic First Aid" and I am unable to replicate the issue you are having. When I selected the product and changed the quantity drop down fields, the total amount is reflected correctly. Please check the screenshot below:
Have you embedded your form in your web page where it is not working? Please share the web page URL and we will take a look.
Sometime if there is a code conflict between your form script and other scripts of your web page, the calculation does not work. Embedding your form with its iFrame embed code will solve the problem. The following guide should help you grab your form's iFrame embed code: https://www.jotform.com/help/148-Getting-the-Form-iFrame-Code
We will wait for your response.