Not the same at all. The Postal Inspectors are law enforcement, not attorneys. They don't do prosecutions at all, the court stuff is handled by the regular Federal Attorney's who work for the Attorney General. The Postal Inspectors are basically a specialized FBI, but like the FBI, once the cuffs are on the suspect it's out of their hands. Though they may be called to testify, prosecution decisions are made by a different group responsible to different people- literally the first point of overlap in the chain of command between a Federal Attorney and a Postal Inspector is the President of the United States (the FBI, as also part of the Department of Justice, overlaps at the Attorney General).

I imagine that in the US, the actual prosecution is handled by US Attorneys though (the DOJ), and not the Post Office