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Though is it not possible to conditionally change the shipping amount based on the selection of quantity but you can add option in your payment question with special pricing to achieve your requirement.
Here are the steps you should follow:
1. Add a standard product in your payment question and add the quantity options:
2. Enable the special pricing by checking the check box and then click next:
3. Set the pricing for every quantity:
The following guides should help you:
Special Pricing: https://www.jotform.com/help/265-What-is-Special-Pricing
Please take a look at the following form and see if this works with you. This is not an ideal solution but is very close to your requirement: http://form.jotformpro.com/form/41560908565965?
Feel free to clone this form for a closer look. The following guide should help you in form cloning: https://www.jotform.com/help/42-How-to-Clone-an-Existing-Form-from-a-URL
Hope this helps.
Do get back to us if you have any questions.