It is a best practice to create an Envelope for each Export Package. This is because documents are only added to the package upon import, re-indexing, or other document change events (such as re-ordering pages). In order to add existing documents that have not changed to the package, you must add them to an Envelope that is configured to be included in the package.
The Document Types contained in the Envelope must match the Document Types configured for the package. The contents of an Envelope are only included in a package if all of the documents in the Envelope belong to Document Types configured to be included in the package. Furthermore, if a package requires a certain Document Type, the Envelope must contain at least one document of that Document Type for the Envelope to be included in the package.