Hi Jenela,
Yup u r solution worked 
..
I have created custom routine and did same what u said.
Now output type is triggering only if posting status is 'C'.
I have created invoice using VF01 , postins status is in A ( This time output is
not Triggered).
Then I ran Transaction VFX3 and then status changes to 'C' . (After this output
is Triggered).
This is worked for above scenario.
let me show this to user and will get back to you.
Thanks a lot for all u r valuable infomation.
One more doubt , will custome routine will each hit each time when posting
status of invoice changed to 'C' .
I mean to say there might be many ways like workflow , vfx3 etc etc to change
posting status of invoice to 'C' correct ( Please correct me if I am wrong).
so will it be work for each time?
Regards,
Mayur