Cartman's mom was given Mr. Slave's package.
|Don't gaze at it too long!
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Go to the post office and get Mr. Slave's package for him.
The post office is found on the main street, by Tom's Rhinoplasty. Go inside and head to the Postal Employees Only gate. Open it and retrieve the package in the back. It is bouncing and vibrating. Bring it back to Mr. Slave's house and he will give you his summon spell to help you in non-boss battles.