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

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?

"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]