Should I use "could" or "should" in this sentence?

[azhu]

I was practicing how to use "can, could, may, might, must, should..."

This question confused me:"It hasn't arrived yet. The courier ___ be delayed in the traffic."

I thought "could" and "should" were both fit in this sentence, but the answer was only "could"...Why was that? Aren't "could" and "should" expressing possibility?

[Calis]

"Should" wouldn't make sense in this case, as it is used to denote that something has a "weak" obligation to be in a certain state, or an expectation that this something is in a certain state.

The above sentence with "should" would only make sense if the person talking was trying to delay the delivery of a package, but that is quite contrived.

[Not a teacher]