Excellent questions. It is free unless you want to use custom fields. The emails go into a queue which is listened for and processed when items are added. I have 3 that listeners that run simultaneously to handle email sending.
If the site goes down then the emails are lost. I'm still working on a solution of multi region load balancing with high availability to ensure this doesn't happen.
This is not a problem for formpost. The alias is all you need to get your form. If you have a contact form on each blog post you will receive a submitted form regardless of URL.
Thank you! Right now during this phase we don't have any imposed sending limits, but we do have a soft cap of 200 emails per hour. This is simply to combat against spam. If someone is sending 200+ emails per hour they should probably find a different solution to be honest. That cap could change depending on behavior and usage as we monitor the service.
Future plans to monetize may include saved messages (which I'm not to keen on), multiple alias' or alias forwarding, the custom fields may or may not be monetized. I may go the Patreon route and if people like the service they can pay what they want.
I am the creator of https://formpost.co. This is just a free microservice from https://darovi.co for people to use with their static websites. There are many like it, but this one is mine ;)
We will be adding custom fields in January 2017 so feel free to give it a try and thank you.