Allow other resources to set/clear postal route

[NotLazy]

This PR allows other resources to essentially "execute" the /postal command, via an export.

It achieves this by extrapolating the /postal logic out into a function and adding an export that calls that function, while replacing the postal logic with a call to that new function.

This PR should be compatible with my other PR, but let me know if you have any trouble merging and i'll happily resolve any conflicts my branch has.