You can clone with
HTTPS or Subversion.
using the web-admin it is possible to add gratis items to an invoice using 0.00 as price identifier.
When using the API to add an item to an invoice this does not work. It seems to require at least 0.01 as price to update the item, otherwise the item doesn't show up in the invoice.
Fix for this issue will be released with new version
Issue fixed with 2.4.30