Please see the official response from Webflow which resolved the soecific issue for me:
This is a known issue with non-shippable products and the billing address. Prior to adding the non-shippable option, the country field was retrieved from the shipping address for the billing address. You will note within your checkout that the billing address does not have a country selector field, and I believe this is what is causing the error. What you will want to do in this case is copy the country selector field and paste it into the billing address section. You can then also set conditional visibility for the shipping section and the shipping method so those do not show at checkout if the product does not require shipping. We have made sure that all new Ecommerce shops include the country selector field within the billing address.
I’ve created a short video to further clarify: https://share.getcloudapp.com/7Kuygo50