If you're talking about shirts and mugs separately, they should not be sent back (even though the PDA will prompt to send them back)!
If you are talking about them bundled together, than I'd say you should be able to simply add them to the carton as you would any movie with an accessory included (e.g., those Minion blu-rays with Minion glasses included in the packaging).
All <word> shop items--the merchandise set along the back wall of entertainment presently--don't get sent back on their own despite having the same department numbers as CDs/movies.