Shipping Cost is grayed out on the Printing Fields tab of a custom form (e.g. Estimates)

To enable the Shipping Cost checkbox on the Printing Fields tab of a Custom Form, you need to enable the "Charge for Shipping" checkbox on the Set Up Shipping page.

Here are the steps: 
    1. Navigate to Setup > Accounting > Shipping.
    2. Set Charge for Shipping box checked. 
    3. Click Submit.

Note:
- This only applies to custom forms that uses the new layout enhancements that has the following features such as "Field Groups, Quick View, etc. whenever a user edit a custom form.