Unexpected business?

In this particular case you might say this, especially if there nothing too pleasant to you about fetching that parcel.

• I have (to see about) some unexpected (little) task.

You might also use the term "personal errand".

(ref;) I need to run a personal errand and will be back for my two-thirty meeting.”

personal errands: noun. a short and quick trip to accomplish a specific purpose, as to buy something, deliver a package, or convey a message, often for someone else. the purpose of such a trip: He finished his errands.

A more general term that would still be specific enough seems to be missing; "chore" implies something you do regularly; I dont think it is proper in this context. There is, otherwise, the word "something", which is general enough, maybe too general and banal, but apparently the most useful; however, you have to add a verb (to do, to see about, to take care of, etc.).

• I have something unexpected to do.

(ref.) Did that extend to the other boys on the team too? We finished cleaning up and opened a portal home. “I need to take care of something this evening, but I'll stop by later.