Hi Milton,
A very good day. 🙂
Yes, you are correct, the option ‘Only the required products have to be in cart’ should work in your case.
I have tested this option with the latest versions of plugins and for me it is working fine. The custom shipping option is showing up if all the products in the cart belongs to that specific category.
To help you with this we will need few more details about your site. We are emailing you shortly with our requirements. Kindly revert back there.
Regards,
Dhara