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Your current configuration that you setup looks defenceless against abusive actions in my opinion. You have all set a default value for the prices of your products but they are still allowed to be changed. You need to make those fields read only:
I also suggest you to use "Number" fields instead of "Textbox" fields since non-numeric data is available to be entered to text boxes. This should create a conflict during the calculation process.
You can also set a range for the quantity field. Please see how:
For the calculation part, please how I set it up on a test form of mine:
You can also clone this form of mine and build your form upon it. Here is the URL of it:
Please see how you can clone it:
However, I strongly suggest you to use a payment field instead of dealing with calculations. You can setup a beautiful product list by using a payment field.
Please read our guide about how to create an order form:
You can add quantities, options and even attach a photo of your product.
Please try these and let us know if you have any difficulties during the configuration process.
Thank you for contacting us.