Thank you for reaching out to Jotform Support. As a workaround, it is possible when you construct your products using regular fields so you can add discounts in the total calculation. Let me show you how to do it :
Use a Multiple choice element for your product items.
Add a value for each Product to be used in the calculation.
You can find my demo form here. If you would like to take a look closer, feel free to clone my form to your account to get access to it. Click here to see how to clone the form.
Give it a try and let us know if you have any other questions.