Hello,
Thanks for the credentials.
I was able to test your website and verify that the plugin "custom field price" is working fine :
pasteboard.co/JEbdSUa.png
For my test, I enabled the order custom field, but I set-up some ACL so it is not visible for guest accounts.
I also published the instance for your "custom field price" plugin, so it can add the price when the checkbox is checked.
The main problem you have is that the "fields" blocks is not automatically submitting its data when you check the box.
I had to manually call a JavaScript function to force the submission of the block, which added the additional line in the cart and refreshed the cart block.
To change that, you need to enable the setting "show submit button" in your checkout workflow so your customers will have a button to validate the content for the fields you have configured.
Auto-submitting fields in the checkout is not possible due to the complexity of some fields and the fact that it could refresh the block when the customer is still performing modifications.
When you do not have the submit button, the "next" button at the bottom will submit all the form and should stay on the same page but with the modifications for the fields and the update of the cart ; but the best is to display the button for the fields block.
Regards,