In addition to the excellent advice received from Mr. Ansari...
Another option would be to have a polling page running on the garage side that checks the status of an application scoped object.
The Office side would write a message
string to the object.
The garage side would periodically check for new messages in the same object.
If there is a new message, create a popup on the garage side and remove or mark the message as read.
Either javascript or a meta refresh tag could trigger the polling.