Are you on a Mac by chance?
Mac’s Archive Utility detects that the .unityPackage within the zip is itself a form of a zip file, and it expands the contents for you automatically. While the feature is handy in some instances, when it comes to a zipped .unityPackage it is a definite problem.
Fortunately, there’s a solution, which is outlined here: Mac users solution: No .unitypackage asset in the zip file attached
TLDR: Open Archive Utility, Uncheck option “Keep expanding if possible”