How to make calculation fields read only?

The PayPal fields are read-only but the fields that drive the PayPal payment (and other payments such as bank transfer) are not. Is there a way to make fields visible but read-only to users, while still letting our logic write into them according to the choices made by the user?

Yes it is possible to show the fields in form and make them read only as well to ensure users are not able to input in those fields. Please check the screenshot below on how to show form fields and make it read only:

Short text entry field:

Form Calculation widget: 

Please take a look at the following cloned form and see if it works as expected:  https://form.jotform.com/91179293490970  

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 

Do get back to us if you have any questions.