The Service API can't handle serializable carts, which, I guess, is technically not a bug though, as it is stated in the docs.

The Content Delivery API can handle them, but does not return any (custom /  additional) properties like the service api does.

So either the Service API should be able to handle serializable carts IMHO... Or the Content API should return custom / additional properties on carts / orders / lineitems. Preferrably both :)