The second invoicing comes into play, that's going to be up to the billing software to determine if they've paid or not, and to decided if the suspension should still happen.
As far as DA knows, they've paid for X bandwidth per month, they've used it all, so they don't get anymore until the next month, at which point they're unsuspended because that falls into the next months usage.
If the client wants to be unsuspended for the current month, then once you get paid the overdue bit, then you'd use the "Additional Bandwidth" field to give them a little bit more.
If you're referring to the API doing the suspension, that should always stick.
Only DA suspension (for bandwidth mainly) will ever be undone.
The difference is set in the user.conf as field:
account=ON | OFF
where a DA suspension sets suspended=yes and leaves account=ON.
If you suspend via the API, it would be suspended=yes and account=OFF.
DA won't unsuspend an "account=OFF" User.
John