How I scaled Postbacks.io to send over 250k HTTP requests per minute

Hi all, I recently launched Postbacks.io to solve a unique problem I encountered with another startup. I simply wanted to receive a callback at a particular time (not recurring) so I could run some task. In my case, this was sending a postcard when a user chose to do so.

Please leave any feedback you have! I'd love to hear how you could use this, or what needs to be improved.