Part of a now defunct fan project, these were available to purchase for a short while. All profits from them were and will be donated to Toys For Tots.
"Your delivery is guaranteed, assuming our mailpony can find the address."
What started out as a simple animation error for less than a second, turned into an indisputable fan favorite, so much so that even the creators took notice. Hell, I was there when her whole mythos began, I had to do Derpy justice. The overall shoe is a bit awkwardly oriented compared to others, but given the nature of the character, it seemed fitting. Since her mane has no real design in it, the tongue is a matching flat yellow. That's counterbalanced by a clean smooth gray, and a bold yellow trim to really boost the look of it. In what has become her fan-official background, Derpy is the diligent mailmare of Equestria, so the heel of these shoes has an image of an envelope, stamped and addressed. The whole "plain design up front/eye catch design at back" feel was very backwards by comparison, but the spirit of it felt right, and it was a popular design.
And coincidentally, these were the only shoes facing backwards in the image preview. Naturally, it was to show off the heel design, but when you think about it, that's kind of fitting don't you think?