Which of the following is an example of an outside parcel?

An outside parcel is typically defined as a package that exceeds the maximum dimensions specified for certain types of mail. In the context of the options provided, a tube that is more than 26 inches long qualifies as an outside parcel because it exceeds the standard size limits for most classes of mail. Outside parcels are often subject to different handling and pricing structures within postal services due to their larger size and irregular shapes.

In contrast, postcards, priority envelopes, and flat-sized packages are generally within the conventional dimensions and weight limits set for standard mail options. Postcards are specifically designed to fit within a certain size range, priority envelopes are standardized for mailing items flat, and flat-sized packages do not exceed the maximum dimensions defined for that category. Therefore, the correctly identified example of an outside parcel is one that exceeds these constraints, highlighting the unique characteristics of outside parcels in postal classification.