Post office employer id number or federal Id ?

[DoninGA]

Which tax form?  If this is for a W-2 then you enter what is on the W-2, nothing else.

[LeonardS]

Some W-2's may show the Employers Identification Number (EIN)  as "employer ID" or "federal ID No".  What it is called is not important so long as it is in the format  xx-xxxxxxx.  Use this number regardless of what it is called.