Hello,
Sadly the team realised it was too much work for them, so this “mail-in your phone” service might never come.
Instead, the dev team has released this week a tool named “Easy Installer” to flash easily a device. For now only a few devices are supported and it’s only available on GNU/Linux.
And no need to root the phone in order to flash a custom ROM. You need to unlock the bootloader and that’s what can’t be done on a lot of US phones.