Custom CSS not working

Hello!
I am using the free version of the wordpress plugin ( https://wordpress.com/plugins/email-template-customizer-for-woo ) and am having issues with applying the custom CSS on the email template. I have added the css to the custom CSS text area and
1) it only sometimes shows on the preview on the customisation page
2) never shows on the email or the separate preview modal.
The css I added is:
.viwec-product-price { padding: 30px ; }

How can I get the custom CSS to ctually apply to my emails?