Hello Andrew,
Thank you for reaching out, and apologies for the inconvenience.
At the moment, there isn’t an option to add a new custom field to the checkout form, including below the “Shipping/Billing Address” section. However, regarding the “VAT ID Number” field you mentioned — it is not limited to 11 characters on our end. According to our development team, the current limit is up to 50 characters, which should accommodate a 16-character alphanumeric tax code without issue.
Could you please recheck this on your site? If you’re still encountering the limitation, kindly share your site URL so we can investigate the issue more closely.
Thank you for your cooperation.
Best regards