Situation
When you must send a value that isn't in the accepted list, a catch-all value that "takes anything" is tempting.
The trap
That catch-all may be a value that turns a feature off. Here it meant "no tracking information" — the request passes, but the end user loses tracking entirely. In that state the record also cannot be corrected from the admin UI.
★ An API accepting a value does not mean the value is semantically right.
★ **When choosing a fallback, look at what that value turns *off*, not what it turns on.**
What was done instead
Carriers missing from the list were identified by tracking-number format and mapped to the closest real code. The option that kept tracking alive was chosen.