ffutures | Fictional postal robbery

I'm trying to identify an old novel about a mail van robbery in New York. Basically, a group of criminals hijack a postal delivery and kidnap several postmen, getting away with a lot of money, checks, etc. The case is investigated by the US Postal Service as well as police etc. I don't think it's a particularly notable book, but I'm writing something that needs a fictional postal inspector, it'd be interesting to see how the service works if I can get hold of a copy.

Ring any bells with anyone?