The cashier called after me because my ex-girlfriend left her wallet.
Oh nice, did you go back to get your ex-girlfriend's wallet?
Yes, but I feel like I'm wasting my time returning the wallet. I would rather she just keep crying like a baby and thinking her wallet was gone forever.
Being upset at an ex is normal but you really should return her wallet. If she were the one who was given your wallet would you want her to return it to you? Think about things like that before you do something petty to get back at them.
Whatever I bet she didn't give a shit about the wallet either.
You are making assumption about her, you already went through the hassle to get the wallet. It is really not that much more to just give it back to her.