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Unfortunately that is not possible to achieve. If you use option flat rate, shipping will be fix and it is not possible to change it.
Possible workaround would be to set your product outside of payment field, using basic fields and then to perform calculation inside of the form. This way you would be able to use conditional logic Update / Calculate Field and to set shipping as 0 if order is over $60.
Once you perform calculation you can pass it to your payment field.
I have made demo form based on your form Stamp Purchase. Here is my demo form:
Feel free to test it and clone it.
Hope this will help. Let us know if you need further assistance.