Well, first of all, I was talking about writing a tracking code for an order with multiple items… BUT… I have to say that there are also other actions that make visible this warning. Maybe I found the matter: it appears every time I have a single order which includes 2 or more items and I mark it as shipped/paid/refunded and/or I select a postal service and/or I insert a tracking code.
I mean… every time I do something with a multiple order (more items in the unique order), this warning appears.
I hope it is more clear now. Thanks again for patience.