Never, ever stack any packages. jmo It's against the methods and only makes things worse. You'll end up handling everything 2-3 times, tripping over packages/narrow walkpaths, bending down twice as much for everything.
Let the misses go by, and your coworker(s) will grab them and cart them back around/dump them on return belt, or whatever the protocol.
I'm a little surprised a low-level part time supervisor would say otherwise, at least outside of UPS. Inside the operation, hypocritical talk is expected.