Thank you. I made several tests already and it always starts to separate next day lastest.
It works with kneading, but it desnt look nice when you get such separated royal icing
It sounds like @Econlady's (Kate's) suggestion of doing PYO cookies may be your best bet then - have them color in a stenciled design with either edible marker or food colorings. Though I know others ship icing with some degree of success. Perhaps your icing recipe is less stable than others?