Paid?

As me and my mate carried a wardrobe out of DFS this morning the security guard stopped us and said, "Can you prove that you've paid for this?"
"Sure," I replied, "Reach into my left trouser pocket."
"Ok," he said, slipping his hand inside, "Your pocket is empty!"
"Exactly," I said,
"Sure," I replied, "Reach into my left trouser pocket."
"Ok," he said, slipping his hand inside, "Your pocket is empty!"
"Exactly," I said,
Spoiler: