Somepony contact Hasbro immediately! Maybe early in the episode, she buys the shoes. She immediately breaks all four heels. Later, maybe they need STRONG glue (to affix Big Mac to Rainbow Dash for weight training, I dunno, whatever) and they have Derpy using it on said heels when a muffin truck passes after she's only managed to glue one heel on. She gallops, hilarity ensues, glue is suddenly available for Mane 6 use.